erl_docgen: fix code highlighting and warning and note parts in erl_docgen
Revert "Merge branch 'fredrik/doc/syntax_highlight/OTP-11237' into maint"

This reverts commit 1562340596938d1f8a84b2ed43f5f0097be66ae7, reversing
changes made to a5128108ad3efff77c5080f0769737e70a0bfd82.