documentation: improve docs for `checkout_head`
documentation: improve docs for `checkout_head`

`git_checkout_head` is sadly misunderstood as something that can
switch branches.  It cannot.  Update the documentation to reflect this.

stewid/libgit2,Corillian/libgit2,Corillian/libgit2,stewid/libgit2,Corillian/libgit2,stewid/libgit2,Corillian/libgit2,stewid/libgit2,stewid/libgit2,stewid/libgit2,Corillian/libgit2,Corillian/libgit2