docs: Document that the man pages are for our builtins

For builtins that have the same name as common commands, it might not
be entirely obvious that there is another page.

So, for those builtins, we add a note, but only in the man pages.

(exception is true and false because the note would be longer than the
page, and it's fridging true and false)

Fixes #8077.

doc_src/cmds/time.rst

@@ -13,6 +13,12 @@ Synopsis
 Description
 -----------
 
+.. only:: builder_man
+
+          NOTE: This page documents the fish keyword ``time``.
+          To see the documentation on the ``time`` command you might have,
+          use ``command man time``.
+
 ``time`` causes fish to measure how long a command takes and print the results afterwards. The command can be a simple fish command or a block. The results can not currently be redirected.
 
 For checking timing after a command has completed, check :ref:`$CMD_DURATION <variables-special>`.