CMake: clean up docs related to MAGNUM_PLUGINS_DIR.

Now with the plugin paths being autodetected those variables have less
use and are more important during the build than during actual use.

doc/building.dox

@@ -481,6 +481,15 @@ thread-local Magnum contents. This is inherited from the
 @ref CORRADE_BUILD_MULTITHREADED option --- if you are sure that you will never
 need such feature, disable it when building Corrade itself.
 
+Various plugin interfaces by default search for plugins in pre-defined
+locations. By default those are defined to the installation location and can be
+overriden using the `MAGNUM_PLUGINS_DIR`, `MAGNUM_PLUGINS_DEBUG_DIR` and
+`MAGNUM_PLUGINS_RELEASE_DIR` CMake variables --- see also
+@ref Trade::AbstractImporter::pluginSearchPaths() and equivalent functions in
+other plugin interfaces. Another option is supplying the plugin search path
+explicitly on the user side either using the same CMake variables (passed
+through to preprocessor, see @ref cmake) or via other autodetection means.
+
 The features used can be conveniently detected in depending projects both in
 CMake and C++ sources, see @ref cmake and @ref Magnum/Magnum.h for more
 information. See also @ref corrade-cmake and @ref Corrade/Corrade.h for
@@ -638,14 +647,6 @@ There are also a few command-line utilities, also all disabled by default:
 -   `WITH_IMAGECONVERTER` --- Build the @ref magnum-imageconverter "magnum-imageconverter"
     executable for converting images of different formats.
 
-Some of these utilities operate with plugins and they search for them in the
-default plugin locations. You can override these locations using the
-`MAGNUM_PLUGINS_DIR` and `MAGNUM_PLUGINS_[DEBUG|RELEASE]_DIR` variables, much
-like when using Magnum from dependent projects --- see @ref cmake for more
-information. In particular, if you specify them as relative paths, the path
-will be taken relative to executable location, which is useful for making
-relocatable installations.
-
 Note that each [namespace](namespaces.html) documentation, all @ref Platform
 library documentation and the @ref GL::OpenGLTester class documentation contain
 more detailed information about dependencies, availability on particular

doc/cmake.dox

@@ -78,10 +78,7 @@ This module tries to find base Magnum library and then defines:
     library was found
 -   `MAGNUM_PLUGINS_DIR` --- Base directory with dynamic plugins, defaults to
     `MAGNUM_PLUGINS_RELEASE_DIR` in release builds and multi-configuration
-    builds or to `MAGNUM_PLUGINS_DEBUG_DIR` in debug builds. You can modify all
-    three variable (e.g. set them to `.` when deploying on Windows with plugins
-    stored relatively to the executable), the following `MAGNUM_PLUGINS_*_DIR`
-    variables depend on it.
+    builds or to `MAGNUM_PLUGINS_DEBUG_DIR` in debug builds.
 -   `MAGNUM_PLUGINS_FONT[|_DEBUG|_RELEASE]_DIR` --- Directory with dynamic font
     plugins
 -   `MAGNUM_PLUGINS_FONTCONVERTER[|_DEBUG|_RELEASE]_DIR` --- Directory with
@@ -216,7 +213,7 @@ release libraries). Note that this autodetection might fail for the
 `MAGNUM_PLUGINS_DIR` variable, especially on multi-configuration build systems.
 You can make use of @ref corrade-cmake "CORRADE_IS_DEBUG_BUILD" preprocessor
 variable along with `MAGNUM_PLUGINS_*_DEBUG_DIR` / `MAGNUM_PLUGINS_*_RELEASE_DIR`
-variables to decide in preprocessing step.
+variables to decide in preprocessing step instead.
 
 Features of found Magnum library are exposed in these CMake variables, they
 are also available as preprocessor variables if including

modules/FindMagnum.cmake

@@ -25,9 +25,7 @@
 #  MAGNUM_PLUGINS_DIR           - Base directory with dynamic plugins, defaults
 #   to :variable:`MAGNUM_PLUGINS_RELEASE_DIR` in release builds and
 #   multi-configuration builds or to :variable:`MAGNUM_PLUGINS_DEBUG_DIR` in
-#   debug builds. You can modify all three variables (e.g. set them to ``.``
-#   when deploying on Windows with plugins stored relatively to the
-#   executable), the following ``MAGNUM_PLUGINS_*_DIR`` variables depend on it.
+#   debug builds
 #  MAGNUM_PLUGINS_FONT[|_DEBUG|_RELEASE]_DIR - Directory with dynamic font
 #   plugins
 #  MAGNUM_PLUGINS_FONTCONVERTER[|_DEBUG|_RELEASE]_DIR - Directory with dynamic