doc: document size suffix for flux-restore --size-limit options

Problem: The use of [kKMG] in the SIZE argument of the flux-restore(1)
--size-limit option is not documented.

Amend the documentation of this option to describe the use of the
optional suffixes.

Author: grondo

doc/man1/flux-restore.rst

@@ -54,7 +54,10 @@ OPTIONS
    store.  Performance will vary depending on the content of the archive.
 
 **--size-limit**\ =\ *SIZE*
-   Skip restoring keys that exceed SIZE bytes (default: no limit).
+   Skip restoring keys that exceed SIZE bytes (default: no limit). SIZE may
+   be specified as a floating point number with an optional multiplicative
+   suffix k or K=1024, M=1024\*1024, or G=1024\*1024\*1024 (up to
+   ``INT_MAX``).
 
 RESOURCES
 =========