Help: Refine RULE_LAUNCH_{COMPILE,CUSTOM,LINK} global property docs

Link to the generators that implement these properties.  Explain in
more detail why other generators ignore them.

Help/prop_gbl/RULE_LAUNCH_COMPILE.rst

@@ -3,7 +3,9 @@ RULE_LAUNCH_COMPILE
 
 Specify a launcher for compile rules.
 
-Makefile generators prefix compiler commands with the given launcher
-command line.  This is intended to allow launchers to intercept build
-problems with high granularity.  Non-Makefile generators currently
-ignore this property.
+:ref:`Makefile Generators` and the :generator:`Ninja` generator prefix
+compiler commands with the given launcher command line.
+This is intended to allow launchers to intercept build problems
+with high granularity.  Other generators ignore this property
+because their underlying build systems provide no hook to wrap
+individual commands with a launcher.

Help/prop_gbl/RULE_LAUNCH_CUSTOM.rst

@@ -3,7 +3,9 @@ RULE_LAUNCH_CUSTOM
 
 Specify a launcher for custom rules.
 
-Makefile generators prefix custom commands with the given launcher
-command line.  This is intended to allow launchers to intercept build
-problems with high granularity.  Non-Makefile generators currently
-ignore this property.
+:ref:`Makefile Generators` and the :generator:`Ninja` generator prefix
+custom commands with the given launcher command line.
+This is intended to allow launchers to intercept build problems
+with high granularity.  Other generators ignore this property
+because their underlying build systems provide no hook to wrap
+individual commands with a launcher.

Help/prop_gbl/RULE_LAUNCH_LINK.rst

@@ -3,7 +3,9 @@ RULE_LAUNCH_LINK
 
 Specify a launcher for link rules.
 
-Makefile generators prefix link and archive commands with the given
-launcher command line.  This is intended to allow launchers to
-intercept build problems with high granularity.  Non-Makefile
-generators currently ignore this property.
+:ref:`Makefile Generators` and the :generator:`Ninja` generator prefix
+link and archive commands with the given launcher command line.
+This is intended to allow launchers to intercept build problems
+with high granularity.  Other generators ignore this property
+because their underlying build systems provide no hook to wrap
+individual commands with a launcher.