Documentation/git-update-ref.txt: remove confusing paragraph

Documentation/git-update-ref.txt: remove confusing paragraph

This paragraph interrupts the flow of the section by going into detail
about what a symbolic ref file is and how it is implemented.  It is not
clear what the purpose is since symbolic refs were already mentioned
prior (“possibly dereferencing the symbolic refs”).  Worse, it can
confuse the reader about what argument can be a symbolic ref since it
just says “it” and not which of the parameters; in turn the reader can
be lead to try `<new-oid>` and then get a confusing error since
update-ref will just say that it is not a valid SHA1.

gitglossary(7) already documents what a symref is, concretely, and quite
well at that.

Reported-by: Bence Ferdinandy <bence@ferdinandy.com>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Taylor Blau <me@ttaylorr.com>