NFSD: Document reply_cache_stats ABI

/proc/fs/nfsd/reply_cache_stats has been present since v3.10 but
has no entry in Documentation/ABI/. Add one under testing/ that
documents the current field set, types, and parsing expectations.

This establishes a contract that parsers should match on field
name rather than line position, allowing fields to be added or
removed across kernel versions without breaking well-written
consumers.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: NeilBrown <neil@brown.name>
Link: https://patch.msgid.link/20260717001232.438792-6-cel@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>

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