It's generally a good idea to avoid trailing whitespace in order to keep
patchs minimal. While it's common to enforce such restrictions for C code,
it's important for docbok XML files too. Hence, let's clean this up and
remove all trailing whitespace currently in place.

[By policy we do not clean up historical trailing whitespace and
tab-indentation in the C source code unless we are modifying those lines
anyway, to retain the ability to merge stable-branch bugfixes into the
development branch. However, the copy of the spec in the development
branch is the only one that receives any updates, so that concern
doesn't apply here. -smcv]

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=91755