remove_definitions: Update documentation

- Reworded command description.
- Briefly mentioned that this command can also remove other flags given
  by add_definitions().
- Added examples section.
- Mentioned also add_compile_definitions() command, as
  remove_definitions() can remove also those.

Help/command/remove_definitions.rst

@@ -1,11 +1,32 @@
 remove_definitions
 ------------------
 
-Remove -D define flags added by :command:`add_definitions`.
+Removes compile definitions added by :command:`add_compile_definitions`, or
+:command:`add_definitions`:
 
 .. code-block:: cmake
 
-  remove_definitions(-DFOO -DBAR ...)
+  remove_definitions([<definitions>...])
 
-Removes flags (added by :command:`add_definitions`) from the compiler
-command line for sources in the current directory and below.
+The arguments are:
+
+``<definitions>...``
+  Zero or more compile definitions.
+
+This command can be also used to remove any flags added by
+:command:`add_definitions`, but it is intended to remove preprocessor
+definitions passed with ``-D``, or ``/D``.
+
+Examples
+^^^^^^^^
+
+In the following example targets of the current directory scope will have
+only ``BAZ`` and ``QUUX`` compile definitions:
+
+.. code-block:: cmake
+
+  add_compile_definitions(FOO BAR BAZ -DQUUX)
+
+  # ...
+
+  remove_definitions(-DFOO -DBAR)