Clarify the confusing "(text)" annotation in the character lists.

Clarify the confusing "(text)" annotation in the character lists.
In some cases, it meant "render as an ASCII character in output
modes that have a notion of codepoints" (e.g. UTF-8, HTML); in other
cases, "render in the text font in output modes that also provide
a special font for mathematical symbols" (e.g. PostScript, PDF).
Also explicitly annotate the escape sequences that use a special
font if available.
OK bentley@