docs: add is_vendor to functions (#850)

docs: add is_vendor to functions

Author: ajnavarro

docs/using-gitbase/functions.md

@@ -8,6 +8,7 @@ To make some common tasks easier for the user, there are some functions to inter
 |:-------------|:-------------------------------------------------------------------------------------------------------------------------------|
 |`is_remote(reference_name)bool`| check if the given reference name is from a remote one                                                          |
 |`is_tag(reference_name)bool`| check if the given reference name is a tag                                                                         |
+|`is_vendor(file_path)bool`| check if the given file name is a vendored file                                                                         |
 |`language(path, [blob])text`| gets the language of a file given its path and the optional content of the file                                    |
 |`uast(blob, [lang, [xpath]]) blob`| returns a node array of UAST nodes in semantic mode                                                          |
 |`uast_mode(mode, blob, lang) blob`| returns a node array of UAST nodes specifying its language and mode (semantic, annotated or native)          |