Add documentation on Mapping=tex-text

[PanderMusubi]

The documentation on Mapping=tex-text is minimal. Please explain the impications of using it. Describe not only what it enables but also what it disables. For example an apostrophe ' does not result in one with this option. Here are some good examples https://www.cl.cam.ac.uk/~mgk25/ucs/quotes.html which can be extended with --- for — mdash.

Experienced users of TeX or people working with exising TeX documents will be interested in this option. However, new users or new documents completely in UTF-8 should avoid this option. Especially with compose key sequences it is easy to create documents with UTF-8 and a minimum of TeX commands. Reusing parts of the content is easier since no escaping or unescaping is needed.

For me, the advantages and disadvantages are slowly becoming clearer. An experienced user could probably draw up a list of common pros and cons for both modes.