doc: document flux-dump's new --maxreqs option

Problem: The new --maxreqs option in flux-dump is not documented.

Add it to flux-dump(1).

Author: chu11

doc/man1/flux-dump.rst

@@ -76,6 +76,14 @@ OPTIONS
    store.  This may be slightly faster, depending on how frequently the same
    content blobs are referenced by multiple keys.
 
+.. option:: --maxreqs=N
+
+   Speed up :program:`flux-dump` by increasing the number of
+   concurrent lookup requests that can be made.  When the dump is
+   large, this could create a heavy burden on the KVS and should not
+   be used unless the system is quiescent or it is an emergency.
+   Defaults to 1 (i.e. no concurrency).
+
 .. option:: --ignore-failed-read
 
    If KVS metadata is encountered that references nonexistent blobrefs

doc/test/spell.en.pws

@@ -1005,3 +1005,4 @@ bourne
 histchars
 cpusets
 sproc
+maxreqs