Help: Clarify if(TARGET) behavior (#14646)

Define the meaning of a "target" clearly and link to the commands
that create them.

Help/command/if.rst

@@ -66,7 +66,9 @@ Possible expressions are:
  True if the given name is an existing policy (of the form ``CMP<NNNN>``).
 
 ``if(TARGET target-name)``
- True if the given name is an existing target, built or imported.
+ True if the given name is an existing logical target name such as those
+ created by the :command:`add_executable`, :command:`add_library`, or
+ :command:`add_custom_target` commands.
 
 ``if(EXISTS path-to-file-or-directory)``
  True if the named file or directory exists.  Behavior is well-defined