Documentation/git-update-ref.txt: remove safety paragraphs

Documentation/git-update-ref.txt: remove safety paragraphs

Remove paragraphs which explain that using this command is safer than
echoing the branch name into `HEAD`.

Evoking the echo strategy is wrong now under the reftable backend since
this file does not exist.  And the ref file backend majority user base
use porcelain commands to manage `HEAD` unless they are intentionally
poking at the implementation.

Maybe this warning was relevant for the usage patterns when it was
added[1] but now it just takes up space.

† 1: 129056370ab (Add missing documentation., 2005-10-04)

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Taylor Blau <me@ttaylorr.com>