use faster method to get short_description from DOCUMENTATION (#42705)

* use faster method to get short_description from DOCUMENTATION

* fixed pep8 whitespace

* fixed blank line

lib/ansible/parsing/plugin_docs.py

@@ -80,3 +80,43 @@ def read_docstring(filename, verbose=True, ignore_errors=True):
             raise
 
     return data
+
+
+def read_docstub(filename, verbose=True, ignore_errors=True):
+    """
+    Quickly find short_description using string methods instead of node parsing.
+    This does not return a full set of documentation strings and is intended for
+    operations like ansible-doc -l.
+    """
+
+    data = {
+        'doc': None,
+        'plainexamples': None,
+        'returndocs': None,
+        'metadata': None
+    }
+
+    try:
+        t_module_data = open(filename, 'r')
+        capturing = False
+        doc_stub = []
+
+        for line in t_module_data:
+            # start capturing the stub until indentation returns
+            if capturing and line[0] == ' ':
+                doc_stub.append(line)
+            elif capturing and line[0] != ' ':
+                break
+            if 'short_description:' in line:
+                capturing = True
+                doc_stub.append(line)
+
+        data['doc'] = AnsibleLoader(r"".join(doc_stub), file_name=filename).get_single_data()
+
+    except:
+        if verbose:
+            display.error("unable to parse %s" % filename)
+        if not ignore_errors:
+            raise
+
+    return data