Help: Document target_compile_definitions handling of -D

Add an example of using the `target_compile_definitions` command and a
note about usage of ``-D`` in items.

Help/command/target_compile_definitions.rst

@@ -27,3 +27,13 @@ Arguments to ``target_compile_definitions`` may use "generator expressions"
 with the syntax ``$<...>``.  See the :manual:`cmake-generator-expressions(7)`
 manual for available expressions.  See the :manual:`cmake-buildsystem(7)`
 manual for more on defining buildsystem properties.
+
+Any leading ``-D`` on an item will be removed.  Empty items are ignored.
+For example, the following are all equivalent:
+
+.. code-block:: cmake
+
+  target_compile_definitions(foo PUBLIC FOO)
+  target_compile_definitions(foo PUBLIC -DFOO)  # -D removed
+  target_compile_definitions(foo PUBLIC "" FOO) # "" ignored
+  target_compile_definitions(foo PUBLIC -D FOO) # -D becomes "", then ignored