Added section for ES2-compatibility-only features into documentation.

Doxyfile

@@ -220,6 +220,7 @@ ALIASES                = \
     "extension{2}=<a href=\"http://www.opengl.org/registry/specs/\1/\2.txt\"><tt>\1_\2</tt></a>" \
     "requires_gles30=@xrefitem requires-gles30 \"Requires OpenGL ES 3.0\" \"Functionality requiring OpenGL ES 3.0\"" \
     "requires_gl=@xrefitem requires-gl \"Requires desktop OpenGL\" \"Functionality requiring desktop OpenGL (not available in OpenGL ES)\"" \
+    "requires_gles20=@xrefitem requires-gles20 \"Requires OpenGL ES 2.0\" \"Functionality requiring OpenGL ES 2.0 (not available in ES 3.0 and desktop OpenGL)\"" \
     "requires_es_extension=@xrefitem requires-es-extension \"Requires OpenGL ES extension\" \"Functionality requiring specific OpenGL ES extension\"" \
     "es_extension{2}=<a href=\"http://www.khronos.org/registry/gles/extensions/\1/\1_\2.txt\"><tt>\1_\2</tt></a>" \
     "es_extension2{3}=<a href=\"http://www.khronos.org/registry/gles/extensions/\1/\3.txt\"><tt>\1_\2</tt></a>"

doc/required-extensions.dox

@@ -48,13 +48,11 @@ supported on Intel GPUs even if they are capable of OpenGL 2.1 only).
 - @subpage requires-gl43
 - @subpage requires-extension
 - @subpage requires-gl
+- @subpage requires-gles20
 - @subpage requires-gles30
 - @subpage requires-es-extension
 - @subpage unsupported
 
-@page requires-gl Functionality requiring desktop OpenGL (not available on OpenGL ES)
-@see @ref MAGNUM_TARGET_GLES_ "MAGNUM_TARGET_GLES"
-
 @page requires-gl30 Functionality requiring OpenGL 3.0
 @page requires-gl31 Functionality requiring OpenGL 3.1
 @page requires-gl32 Functionality requiring OpenGL 3.2
@@ -66,9 +64,15 @@ supported on Intel GPUs even if they are capable of OpenGL 2.1 only).
 
 @page requires-extension Functionality requiring specific OpenGL extension
 
-@page requires-gles30 Functionality requiring OpenGL ES 3.0
+@page requires-gl Functionality requiring desktop OpenGL (not available on OpenGL ES)
+@see @ref MAGNUM_TARGET_GLES_ "MAGNUM_TARGET_GLES"
+
+@page requires-gles20 Functionality requiring OpenGL ES 2.0 (not available in ES 3.0 and desktop OpenGL)
 @see @ref MAGNUM_TARGET_GLES2_ "MAGNUM_TARGET_GLES2"
 
+@page requires-gles30 Functionality requiring OpenGL ES 3.0
+@see @ref MAGNUM_TARGET_GLES3_ "MAGNUM_TARGET_GLES3"
+
 @page requires-es-extension Functionality requiring specific OpenGL ES extension
 @see @ref MAGNUM_TARGET_GLES2_ "MAGNUM_TARGET_GLES2"