Docs feedback: Describe how to specify a username string

• Start this issue's title with Docs: or Docs feedback:.

Problem to solve

The following feedback was provided from a user about the Use Git troubleshooting section, specifically the Error on Git fetch: “HTTP Basic: Access Denied” section:

"To resolve this, specify a username string."

It doesn't say how do to so.

Proposal

Update the troubleshooting section so that it is clearer and more actionable for users.

Other links/references

• https://gitlab.com/gitlab-org/gitlab/-/issues/564995