Add doc for description element and update fonts.dtd

Fixes fontconfig/fontconfig#133
2 jobs for <span class="ref-name">issue/133</span> in 5 minutes and 15 seconds (queued for 2 minutes and 26 seconds)
Status Job ID Name Coverage
  Build
passed #44337
shared-build

00:05:15

passed #44338
static-build

00:03:54