doc: usage: Add general rule for $?

For nearly all commands in U-Boot the '?' variable is handled the same
way with 0 meaning success, 1 meaning any failure.  Explain this in the
general rules section of the cmdline documentation (with a link to a
counter example) and then remove the redundant wording from most
commands. We retain a section about the return value in a number of
places where we are doing something such as always returning a specific
value or we have useful additional information to go along with the
normal return codes.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

doc/usage/cmd/pause.rst

@@ -48,9 +48,3 @@ Note that complex prompts require proper quoting:
     
     Usage:
     pause [prompt] - Wait until users presses any key. [prompt] can be used to customize the message.
-
-Return value
-------------
-
-The return value $? is always set to 0 (true), unless invoked in an invalid
-manner.