Update Project snippets page based on initial review

Issue description

This page describes product documentation work that was identified during a documentation review of documentation pages associated with the Create:Editor stage/group.

Page: https://docs.gitlab.com/ee/api/project_snippets.html

Identified items

• Should cross-link from https://docs.gitlab.com/ee/api/project_snippets.html to https://docs.gitlab.com/ee/user/snippets.html and vice versa.
• Add a period at the end of each description in all tables.
• Move Updates to snippets with multiple files must use the files attribute. to the files attribute in the table.