doc: git-checkout: clarify intro sentence
From user feedback: in the first paragraph, 5 users reported not understanding the terms "pathspec" and 1 user reported not understanding the term "HEAD". Of the users who said they didn't know what "pathspec" means, 3 said they couldn't understand what the paragraph was trying to communicate as a result. One user also commented that "If no pathspec was given..." makes `git checkout <branch>` sounds like a special edge case, instead of being one of the most common ways to use this core Git command. It looks like the goal of this paragraph is to communicate that `git checkout` has two different modes: one where you switch branches and one where you just update your working directory files/index. So say that directly, and use more familiar language (including examples) to say it. Signed-off-by: Julia Evans <julia@jvns.ca> Signed-off-by: Junio C Hamano <gitster@pobox.com>