clarify fetch-depth: 0 behavior in the documentation

[Loki-Afro]

clarify fetch-depth: 0 behavior in the documentation.

it is unclear if it will also provide local branch information or not.
if it does not provide local branch information the following code does not work:

          if git rev-parse --quiet --verify ${{ env.git_ref_name }} > /dev/null; then
            echo "branch_exists=true" >> $GITHUB_ENV
          else
            echo "branch_exists=false" >> $GITHUB_ENV
          fi

fetch-depth: 0 reads like --mirror to me without looking into the code.