Update description for firstmatch option (#62896)

##### SUMMARY
This fixes issue https://github.com/ansible/ansible/issues/62862. The edit makes the description for the `firstmatch` option clearer.

The `firstmatch` option was introduced by PR https://github.com/ansible/ansible/pull/33825


##### ISSUE TYPE
- Docs Pull Request

+label: docsite_pr

lib/ansible/modules/files/lineinfile.py

@@ -116,7 +116,7 @@ options:
   firstmatch:
     description:
       - Used with C(insertafter) or C(insertbefore).
-      - If set, C(insertafter) and C(insertbefore) find a first line has regular expression matches.
+      - If set, C(insertafter) and C(insertbefore) will work with the first line that matches the given regular expression.
     type: bool
     default: no
     version_added: "2.5"