Help: Clarify add_compile_options documentation (#15225)

Explain that it affects only targets added after the call.  Fix the link
to the COMPILE_OPTIONS directory propert documentation.  Update the
latter to explain that it is used to initialize the corresponding target
property when a target is created.

Help/command/add_compile_options.rst

@@ -7,15 +7,16 @@ Adds options to the compilation of source files.
 
   add_compile_options(<option> ...)
 
-Adds options to the compiler command line for sources in the current
-directory and below.  This command can be used to add any options, but
-alternative commands exist to add preprocessor definitions
-(:command:`target_compile_definitions` and :command:`add_definitions`) or
-include directories (:command:`target_include_directories` and
-:command:`include_directories`).  See documentation of the
-:prop_tgt:`directory <COMPILE_OPTIONS>` and
+Adds options to the compiler command line for targets in the current
+directory and below that are added after this command is invoked.
+See documentation of the :prop_dir:`directory <COMPILE_OPTIONS>` and
 :prop_tgt:`target <COMPILE_OPTIONS>` ``COMPILE_OPTIONS`` properties.
 
+This command can be used to add any options, but alternative commands
+exist to add preprocessor definitions (:command:`target_compile_definitions`
+and :command:`add_definitions`) or include directories
+(:command:`target_include_directories` and :command:`include_directories`).
+
 Arguments to ``add_compile_options`` may use "generator expressions" with
 the syntax ``$<...>``.  See the :manual:`cmake-generator-expressions(7)`
 manual for available expressions.  See the :manual:`cmake-buildsystem(7)`

Help/prop_dir/COMPILE_OPTIONS.rst

@@ -6,9 +6,9 @@ List of options to pass to the compiler.
 This property specifies the list of options given so far to the
 :command:`add_compile_options` command.
 
-This property is used to populate the :prop_tgt:`COMPILE_OPTIONS` target
-property, which is used by the generators to set the options for the
-compiler.
+This property is used to initialize the :prop_tgt:`COMPILE_OPTIONS` target
+property when a target is created, which is used by the generators to set
+the options for the compiler.
 
 Contents of ``COMPILE_OPTIONS`` may use "generator expressions" with the
 syntax ``$<...>``.  See the :manual:`cmake-generator-expressions(7)` manual