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/cp.rst

@@ -79,9 +79,3 @@ Configuration
 The cp command is available if CONFIG_CMD_MEMORY=y. Support for 64 bit words
 (cp.q) is only available on 64-bit targets. Copying to flash depends on
 CONFIG_MTD_NOR_FLASH=y.
-
-Return value
-------------
-
-The return value $? is set to 0 (true) if the command was successfully,
-1 (false) otherwise.