doc: a link to an anchor Doxygen doesn't know about is a warning now.

Which is good, those are all either anchors in the theme itself, or are
group names for which Doxygen doesn't make anchors anymore (but m.css
does). Plain HTML link still works as before, so let's do that instead.

src/Magnum/Math/Functions.h

@@ -647,9 +647,9 @@ template<std::size_t size, class T> inline Vector<size, T> fma(const Vector<size
 /**
 @{ @name Exponential and power functions
 
-Unlike @m_class{m-doc} [scalar/vector functions](#scalarvector-functions) these
+Unlike @m_class{m-doc} <a href="#scalarvector-functions">scalar/vector functions</a>
-don't work on @ref Magnum::Math::Deg "Deg" / @ref Magnum::Math::Rad "Rad" as
+these don't work on @ref Magnum::Math::Deg "Deg" / @ref Magnum::Math::Rad "Rad"
-the resulting unit can't be easily expressed.
+as the resulting unit can't be easily expressed.
 */
 
 /**

src/Magnum/Math/Packing.h

@@ -36,7 +36,7 @@ namespace Magnum { namespace Math {
 /**
 @{ @name Packing and unpacking functions
 
-Similarly to @m_class{m-doc} [scalar/vector functions](#scalarvector-functions)
+Similarly to @m_class{m-doc} <a href="#scalarvector-functions">scalar/vector functions</a>
 these work on both scalars and vectors, including @ref Magnum::Math::Deg "Deg"
 and @ref Magnum::Math::Rad "Rad".
 */

src/Magnum/ShaderTools/AbstractConverter.h

@@ -331,8 +331,8 @@ concrete converter implementation is loaded and instantiated through a
 @relativeref{Corrade,PluginManager::Manager}. See @ref plugins for more
 information about general plugin usage, @ref file-formats to compare
 implementations for common file formats and the list of
-@m_class{m-doc} [derived classes](#derived-classes) for all available shader
+@m_class{m-doc} <a href="#derived-classes">derived classes</a> for all
-converter plugins.
+available shader converter plugins.
 
 As each converter has different requirements on the source, its format and
 options set, you're expected to perform error handling on the application side

src/Magnum/Text/AbstractFont.h

@@ -118,8 +118,8 @@ all error handling:
 @snippet Text-gl.cpp AbstractFont-usage
 
 See @ref plugins for more information about general plugin usage and the list
-of @m_class{m-doc} [derived classes](#derived-classes) for available font
+of @m_class{m-doc} <a href="#derived-classes">derived classes</a> for available
-plugins. See @ref GlyphCache for more information about glyph caches and
+font plugins. See @ref GlyphCache for more information about glyph caches and
 @ref Renderer for information about actual text rendering.
 
 @subsection Text-AbstractFont-usage-font-size Font size

src/Magnum/Text/AbstractFontConverter.h

@@ -120,8 +120,8 @@ MAGNUM_TEXT_EXPORT Debug& operator<<(Debug& debug, FontConverterFeatures value);
 
 Provides functionality for converting an arbitrary font to different format.
 See @ref plugins for more information and the list of
-@m_class{m-doc} [derived classes](#derived-classes) for available font
+@m_class{m-doc} <a href="#derived-classes">derived classes</a> for available
-converter plugins.
+font converter plugins.
 
 @m_class{m-note m-success}
 

src/Magnum/Trade/AbstractImageConverter.h

@@ -526,8 +526,8 @@ against the reported @ref features() first.
 
 See @ref plugins for more information about general plugin usage,
 @ref file-formats to compare implementations of common file formats and the
-list of @m_class{m-doc} [derived classes](#derived-classes) for all available
+list of @m_class{m-doc} <a href="#derived-classes">derived classes</a> for all
-image converter plugins.
+available image converter plugins.
 
 @m_class{m-note m-success}
 

src/Magnum/Trade/AbstractImporter.h

@@ -242,8 +242,8 @@ In the following example an image is loaded from the filesystem using the
 
 See @ref plugins for more information about general plugin usage,
 @ref file-formats to compare implementations of common file formats and the
-list of @m_class{m-doc} [derived classes](#derived-classes) for all available
+list of @m_class{m-doc} <a href="#derived-classes">derived classes</a> for all
-importer plugins.
+available importer plugins.
 
 @m_class{m-note m-success}
 

src/Magnum/Trade/AbstractSceneConverter.h

@@ -637,8 +637,8 @@ it might be good to check against the reported @ref features() first.
 
 See @ref plugins for more information about general plugin usage,
 @ref file-formats to compare implementations of common file formats and the
-list of @m_class{m-doc} [derived classes](#derived-classes) for all available
+list of @m_class{m-doc} <a href="#derived-classes">derived classes</a> for all
-scene converter plugins.
+available scene converter plugins.
 
 @subsection Trade-AbstractSceneConverter-usage-mesh-data Converting a single mesh data