doc/rados: add kernel client notes to require_min_compat_client

doc/rados: add kernel client notes to require_min_compat_client

Add a "Min kernel" column to the "Features gated by the flag" table:
pg-upmap requires kernel 4.13, pg-upmap-primary and CRUSH MSR are not
yet implemented in the kernel client.

Add a warning noting the distinct failure modes (pg-upmap-primary causes
I/O hangs from silently dropped misdirected ops; CRUSH MSR rules cause
I/O errors because crush_find_rule fails) and that CRUSH MSR rules are
added via osd setcrushmap without require_min_compat_client gating but
immediately raise the features-in-use floor to squid.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>