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/setexpr.rst

@@ -147,10 +147,3 @@ Configuration
 * The *setexpr gsub* and *setexpr sub* sub-commands are only available
   if CONFIG_REGEX=y. For an overview of the supported regex syntax,
   see :doc:`test`.
-
-Return value
-------------
-
-The return value $? is set to 0 (true) if the operation was successful.
-
-If an error occurs, the return value $? is set to 1 (false).