reword documentation for git.autoForwardBranches

[sean-xyz]

• PR Description

The wording "lazygit will automatically forward branches to their upstream after fetching" could be interpreted to mean that lazygit automatically pushes branches, rather than operating locally. Edited to make the behavior more clear.

• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[stefanhaller]

Makes sense, thanks.