@url vs. @uref

[Michael Käppler]

Hi all,
we're using both `@url` and `@uref` commands in our documentation,
the overwhelming majority is `@uref`.

michael] ~/lilypond/Documentation/en (master)]> git grep '@url' | wc -l
9
michael] ~/lilypond/Documentation/en (master)]> git grep '@uref' | wc -l
622

The functionality is exactly the same:
https://www.gnu.org/software/texinfo/manual/texinfo/html_node/_0040url.html

I'd like to unify this. Shall we go with `@uref` to avoid big diffs and
because it is
already the majority, or use `@url` what seems more intuitive to me?

Cheers,
Michael