docs/string: Add a -- example to the match section

This keeps tripping people up. We can't mention it *everywhere*, but
lets see if it works just in "match", since that sees to be where
people hit it most.

doc_src/cmds/string-match.rst

@@ -59,6 +59,12 @@ Match Glob Examples
     >_ string match -i 'a??B' Axxb
     Axxb
 
+    >_ string match -- '-*' -h foo --version bar
+    # To match things that look like options, we need a `--`
+    # to tell string its options end there.
+    -h
+    --version
+
     >_ echo 'ok?' | string match '*\?'
     ok?
 
@@ -91,6 +97,12 @@ Match Regex Examples
     cat4
     dog4
 
+    >_ string match -r -- '-.*' -h foo --version bar
+    # To match things that look like options, we need a `--`
+    # to tell string its options end there.
+    -h
+    --version
+
     >_ string match -r '(\d\d?):(\d\d):(\d\d)' 2:34:56
     2:34:56
     2