documentation: add `zstd` compression to the internal `revlogs` documentation

Differential Revision: https://phab.mercurial-scm.org/D8953

mercurial/helptext/internals/revlogs.txt

@@ -217,6 +217,8 b' encoding of the revision data. The heade'
 
 \0 (0x00)
    Revision data is the entirety of the entry, including this header.
+(   (0x28)
+    zstd https://github.com/facebook/zstd
 u (0x75)
    Raw revision data follows.
 x (0x78)