[FileCheck] Add docs for --allow-empty

This diff adds documentation for `allow-empty` flag under FileCheck
docs.

Reviewed by jhenderson, smeenai, thopre

Differential Revision: https://reviews.llvm.org/D83682

llvm/docs/CommandGuide/FileCheck.rst

@@ -181,6 +181,10 @@ and from the command line.
   as old tests are migrated to the new non-overlapping ``CHECK-DAG:``
   implementation.
 
+.. option:: --allow-empty
+
+  Allow checking empty input. By default, empty input is rejected.
+
 .. option:: --color
 
   Use colors in output (autodetected by default).