Not clear from the documentation whether `--scan-history` scans all the branches

Some people might want to run `git-secrets` on their existing code as a verification, but it is not clear from the documentation how to verify all the branches

Start by reading the documentation for the `--scan-history` option and the guidance on verifying existing code. Clarify whether the scan covers all branches and document how to verify every branch, so users can tell from the documentation what command or process to use.

Written by the indexing model from the issue text.