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>

Documentation/ABI/testing/procfs-nfsd-reply_cache_stats

@@ -0,0 +1,38 @@
+What:		/proc/fs/nfsd/reply_cache_stats
+Date:		March 2013
+KernelVersion:	3.10
+Contact:	linux-nfs@vger.kernel.org
+Description:
+		Provides statistics for the NFS server duplicate reply
+		cache (DRC). The file contains one labeled field per
+		line. Each line has the form "field name:" followed by
+		whitespace and a decimal value.
+
+		Fields:
+
+		=======================  ======  ==========================
+		max entries              u32     Upper bound on cache size
+		num entries              u32     Current entry count
+		hash buckets             u32     Number of hash buckets
+		mem usage                s64     Bytes consumed by the DRC
+		cache hits               s64     Requests answered from cache
+		cache misses             s64     Requests not found in cache
+		not cached               s64     Idempotent requests that
+		                                 bypass the cache
+		payload misses           s64     XID matched but request
+		                                 checksum did not
+		longest chain len        u32     Longest hash chain observed
+		cachesize at longest     u32     Cache size when longest
+		                                 chain was recorded
+		=======================  ======  ==========================
+
+		Counter fields (cache hits, cache misses, not cached,
+		payload misses, mem usage) are maintained with per-cpu
+		counters and may briefly show stale values under
+		concurrent load. There is no way to reset these
+		counters; consumers should compute rates by sampling
+		over time.
+
+		New fields may be appended in future kernels. Parsers
+		should match on field name, not line position.
+Users:		nfs-utils (https://git.linux-nfs.org/?p=steved/nfs-utils.git)