Reference more non-fish shell builtins that have relevant differences

When writing scripts for other shells, it can be confusing and annoying
that our `man` function shadows other manual pages, for example `exec(1p)`
from [Linux man-pages]. I almost never want to see the fish variant for such
contended cases (which obviuosly don't include fish-specific commands like
`string`, only widely-known shell builtins).

For the contented cases like `exec`, the POSIX documentation is more
substantial and useful, since it describes a (sub)set of languages widely
used for scripting.

Because of this I think we should stop overriding the system's man pages.
Nowadays we offer `exec -h` as intuitive way to show the documentation for
the fish-specific command (note that `help` is not a good replacement because
it uses a web browser).

Looking through the contended commands, it seems like for most of them,
the fish version is not substantially different from the system version.
A notable exception is `read` but I don't think it's a very important one.

So I think we should can sacrifice a bit of the native fish-scripting
experience in exchange for playing nicer with other shells. I think the
latter is more important because scripting is not our focus, the way I see it.
So maybe put our manpath at the end.

In lieu of that, let's at least have `exec.rst` reference the system variant.

[Linux man-pages]: https://www.kernel.org/doc/man-pages/

Closes #10376

doc_src/cmds/exec.rst

@@ -13,6 +13,11 @@ Synopsis
 Description
 -----------
 
+.. only:: builder_man
+
+          NOTE: This page documents the fish builtin ``exec``.
+          To see the documentation on any non-fish versions, use ``command man exec``.
+
 ``exec`` replaces the currently running shell with a new command. On successful completion, ``exec`` never returns. ``exec`` cannot be used inside a pipeline.
 
 The **--help** or **-h** option displays help about using this command.