Add USES=tex

[bofh]

Summary

In a recent commit I have changed USE_TEX macros to USES=tex in the ports tree. Till now there were in fact no descriptions of TEX macros at all. So introduce the new macros here in the porters-handbook.

While I am here remove EOL spaces.

Test Plan

Built and serving from here:
https://fdp.bofh.network/books/porters-handbook/uses/

[bcr]

A few small corrections, the rest looks OK to me.
Good to have a TEX section in the porters handbook with this change!

documentation/content/en/books/porters-handbook/uses/_index.adoc

1985

I'd capitalize tex here just like below (abbreviation).
s/adds/add/

1994

s/following/follows/