[Gimp-docs] index terms in visible text

[Vitaly Lomov]

I found index terms (e.g. string Opacity; Brush; Pressure sensibility;
Fade out; Hard edges) getting into html version of the manual. The
following files are affected (for en):
toolbox/tool-airbrush.xml
toolbox/tool-convolve.xml
toolbox/tool-dodge-burn.xml
toolbox/tool-paintbrush.xml
toolbox/tool-pencil.xml
toolbox/tool-smudge.xml

Those terms appear in the final version, break the text flow and are
out of place. What shall we do?