Clarify that f and F show visible links

[gitressa]

Description

Great add-on, it will make browsing so much faster, thanks!

After installing the add-on, I was reading the help page (?) but could not see how to find links. I normally use "search for links in text" via the ' character in Firefox.

https://support.mozilla.org/en-US/kb/search-contents-current-page-text-or-links#w_search-links-only

It was only by mistake that I pressed f and saw the links on the page highlighted with shortcuts, which is an even better solution for finding links.

So would it make sense to clarify that f and F show links on the help page?

PS. I also fixed a few instances where a line starts with a capital letter, since it looks like the style is to only start with small letters.

[allamedo]

@gitressa I think the descriptions at vimium/background_scripts/commands.js should also be updated.

[gitressa]

Thanks @allamedo, I updated the background_scripts/commands.js file as well.