ARM: document DEBUG_UNCOMPRESS Kconfig option

This non-user visible option lacked any kind of documentation.  This
is quite common for non-user visible options; certian people can't
understand the point of documenting such options with help text.

However, here we have a case in point: developers don't understand the
option either, as they were thinking that when the option is not set,
the decompressor should produce no output what so ever.  This is
incorrect, as the purpose of this option is to control whether a
multiplatform kernel uses the kernel debugging macros to produce
output or not.

So let's document this via help rather than commentry to prevent others
falling into this misunderstanding.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>