[DOC] please provide documentation for QML_USE_GLYPHCACHE_WORKAROUND

Please provide documentation for QML_USE_GLYPHCACHE_WORKAROUND environment variable.

It is used when OpenGL driver has a bug when doing FBO readbacks and as a result the rendered glyphs are corrupted. To solve this, Qt needs to keep an additional copy of the glyph cache in RAM (Qt cannot read it back from the graphics hardware to update it when new glyphs are added).

This means that when drawing glyphs that have not been drawn before, performance will be slightly lower since Qt needs to go via the extra
copy on the CPU, and it means the glyph cache will use twice as much memory. Quality is not affected by this.