doc: document undrain -f, --force option in flux-resource(1)

Problem: The `flux resource undrain` `--force` option is undocumented.

Add it to the flux-resource(1) man page.

doc/man1/flux-resource.rst

@@ -17,7 +17,7 @@ SYNOPSIS
 
 | **flux** **resource** **drain** [*-n*] [*-o* *FORMAT*] [*-i* *TARGETS*]
 | **flux** **resource** **drain** [*-f*] [*-u*] [*targets*] [*reason*]
-| **flux** **resource** **undrain** *targets*
+| **flux** **resource** **undrain** [*-f*] *targets*
 
 | **flux** **resource** **reload** [-f] [--xml] *path*
 
@@ -313,6 +313,10 @@ Undrain the nodes specified by the *targets* argument (IDSET or HOSTLIST).
 
 This command is restricted to the Flux instance owner.
 
+.. option:: -f, --force
+
+  Do not fail if any of the *targets* are not drained.
+
 reload
 ------