Apply suggestions from code review

Co-authored-by: Wei Ji <23487320+weiji14@users.noreply.github.com>

Author: Meghan Jones

pygmt/helpers/__init__.py

@@ -4,7 +4,6 @@
 from pygmt.helpers.decorators import (
     deprecate_parameter,
     fmt_docstring,
-    insert_alias,
     kwargs_to_strings,
     use_alias,
 )

pygmt/helpers/decorators.py

@@ -323,9 +323,9 @@ def fmt_docstring(module_func):
     return module_func
 
 
-def insert_alias(module_func, default_value=None):
+def _insert_alias(module_func, default_value=None):
     """
-    Decorator insertings aliases into the signature of a method.
+    Function to insert PyGMT long aliases into the signature of a method.
     """
 
     # Get current signature and parameters
@@ -408,7 +408,7 @@ def new_module(*args, **kwargs):
 
         new_module.aliases = aliases
 
-        new_module = insert_alias(new_module)
+        new_module = _insert_alias(new_module)
 
         return new_module