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/cpu.rst

@@ -93,9 +93,3 @@ Configuration
 -------------
 
 The cpu command is available if CONFIG_CMD_CPU=y.
-
-Return code
------------
-
-The return value $? is set to 0 (true) if the command is successful,
-1 (false) otherwise.