Clarify masked variables docs
Clarify masked variables docs

- Add note that must contain only letters, numbers, underscore
- Add example of escape characters
- Add regex used for validation

closes https://gitlab.com/gitlab-org/gitlab-ce/issues/60785

iiet/iiet-git,stoplightio/gitlabhq,mmkassem/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq,stoplightio/gitlabhq,iiet/iiet-git,mmkassem/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq,iiet/iiet-git,iiet/iiet-git