Documentation/maintainer-tip: Clarify merge window policy

There are lots of maintainers "pings" during the merge window, even
for trivial patches.

Clarify that contributors should not expect progress on *any*
non-urgent patches during the merge window.  This applies to all
contributions, not just large ones.

Clarify the language around -rc1.  Trees really are closed during the
merge window.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/all/20240322183403.67BAEEFE%40davehans-spike.ostc.intel.com