DOC: config: set-var() dconv rendering issues

DOC: config: set-var() dconv rendering issues

Since <cond> optional argument support was added to set-var() and friends
in 2.6 with 164726c ("DOC: vars: Add documentation about the set-var
conditions"), dconv is having a hard time rendering related keywords.

Everywhere `[,<cond> ...]` was inserted, html formatting is now broken.

Removing the space between <cond> and '...' allows dconv to properly parse
the token thus restores proper formatting without changing the meaning.

This was discovered when discussing about var() sample fetch doc issues
in GH #2087

This patch should be backported up to 2.6