doc: document flux resource watch

Problem: The new flux resource watch subcommand is not documented.

Add documentation in flux-resource.rst.

doc/man1/flux-resource.rst

@@ -165,6 +165,13 @@ COMMANDS
    *-f,--force*, resources may contain invalid ranks.  This command is
    primarily used in test.
 
+**watch** [-a] [-r]
+   Output changes in resources from *resource.eventlog* to standard out as they
+   occur.  May be useful when needing to see resource changes in real time.  If
+   *-a,--all* is specified, all output all resource changes since the beginning of
+   the instance.  If *-r,--ranks* is specified, ranks of resources will be output
+   instead of hosts.
+
 
 OUTPUT FORMAT
 =============