Better document sysroot.

All non trivial users of FontConfig must use FcConfigGetSysRoot to
resolve file properties in patterns. In order to support sysroot the
filename in the file property must be relative to the sysroot, but the
value of the file property in a pattern is directly exposed, making it
impossible for FontConfig to resolve the filename itself transparently.
2 jobs for <span class="ref-name">bungeman-sysroot</span> in 7 minutes and 20 seconds (queued for 52 seconds)
Status Job ID Name Coverage
  Build
passed #124256
shared-build

00:07:20

passed #124257
static-build

00:05:59