Help: Reference add_compile_options from add_definitions (#15225)

When we explain in add_definitions documentation that it is intended
only for preprocessor definitions, link to add_compile_options for
adding other flags.  Also explicitly mention that the order of
add_definitions calls with respect to target creation does not matter.
This differs from the behavior now explicitly stated in the
documentation of add_compile_options.

Help/command/add_definitions.rst

@@ -7,10 +7,12 @@ Adds -D define flags to the compilation of source files.
 
   add_definitions(-DFOO -DBAR ...)
 
-Adds definitions to the compiler command line for sources in the current
-directory and below.  This command can be used to add any flags, but
-it is intended to add preprocessor definitions.  Flags
-beginning in -D or /D that look like preprocessor definitions are
+Adds definitions to the compiler command line for targets in the current
+directory and below (whether added before or after this command is invoked).
+This command can be used to add any flags, but it is intended to add
+preprocessor definitions (see the :command:`add_compile_options` command
+to add other flags).
+Flags beginning in -D or /D that look like preprocessor definitions are
 automatically added to the :prop_dir:`COMPILE_DEFINITIONS` directory
 property for the current directory.  Definitions with non-trivial values
 may be left in the set of flags instead of being converted for reasons of