Help: Clarify that find_package GLOBAL argument is supported in full signature

This was accidentally left out of the docs by commit 2f1ffa003c
(find_package: Add support for default GLOBAL imported targets,
2022-03-10).

Help/command/find_package.rst

@@ -162,6 +162,7 @@ Full Signature
                [OPTIONAL_COMPONENTS components...]
                [CONFIG|NO_MODULE]
                [NO_POLICY_SCOPE]
+               [GLOBAL]
                [NAMES name1 [name2 ...]]
                [CONFIGS config1 [config2 ...]]
                [HINTS path1 [path2 ... ]]