doc, gnu, guix: Fix typos.

doc, gnu, guix: Fix typos.

* doc/guix.texi: Fix various typos and reword a sentence.
* gnu/packages/cpp.scm (cpplint)[description]: Fix typo.
* gnu/packages/dns.scm (ldns): Fix typo in comment.
* gnu/packages/games.scm (yamagi-quake2): Fix typo in comment.
* gnu/packages/qt.scm (python-pyqtwebengine): Fix typo in comment.
* gnu/packages/rails.scm (ruby-autoprefixer-rails): Fix typo in comment.
* gnu/packages/syndication.scm (quiterss)[description]: Fix typo.
* gnu/packages/terminals.scm (kmscon): Fix typo in comment.
* gnu/packages/xml.scm (perl-xml-sax-base)[description]: Fix typo.
  (python-elementpath)[description]: Fix typo.
* guix/build/asdf-build-system.scm (output-translation): Fix typo in docstring.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>