magit-{checkout,branch-{create,delete,rename}: improve doc-strings

Improve git usage info in doc-strings, and another
minor adjustment to `magit-branch-delete's doc-string.
magit-{checkout,branch-delete}: cosmetics
magit-checkout: more appropriate prompt

Use "Checkout: " in the prompt, not "Switch to: ", we are git users,
we know what checkout means.  Also don't mention the branch we are
currently on.  That isn't totally irrelevant, but providing this
hint here but not anywhere else, is nothing but confusing.
Merge branch 'n/checkout-branch-commands' into next