add clarification in theming guide about missing glyphs [skip ci]
add clarification in theming guide about missing glyphs [skip ci]

- explain that characters will be missing if the font doesn't provide the glyph
- document that there is no glyph missing warning when using a TrueType font

asciidoctor/asciidoctor-pdf,dsisnero/asciidoctor-pdf,mojavelinux/asciidoctor-pdf,asciidoctor/asciidoctor-pdf,Hextremist/asciidoctor-pdf,dsisnero/asciidoctor-pdf,mojavelinux/asciidoctor-pdf,Hextremist/asciidoctor-pdf