doc: document kvs max ops configs

Problem: The new kvs transaction-max-ops configuration
option is not documented.

Add documentation to flux-config-kvs(5)

Author: chu11

doc/man5/flux-config-kvs.rst

@@ -29,6 +29,13 @@ gc-threshold
    point. (Default: garbage collection must be manually requested with
    `flux-shutdown --gc`).
 
+transaction-max-ops
+   (optional) Sets the maximum number of transactions that can be
+   performed in a single KVS commit.  This configuration is to prevent
+   a single transaction from taking up too much of the KVS's time
+   (i.e. to prevent a denial-of-service from a large transaction).  By
+   default the maximum is 131072.
+
 
 EXAMPLE
 =======

doc/test/spell.en.pws

@@ -948,3 +948,4 @@ backoff
 unkillable
 Feitelson
 TimeoutStopSec
+KVS's