[tldoc] more doc changes

[Karl Berry]

I had to make more small doc changes today, since the SOURCE_DATE_EPOCH
stuff applies to all engines except luatex (next year) and original tex
(intentionally disabled).  I remade the pdf and html again.

As usual, comparing against archive/live-2015.tex is the most reliable
way to see all changes since last year, but here are the specific tweaks
I just committed (Siep's are still there too, of course).

I sincerely hope this will be the end of the doc changes for this year;
at least, I have nothing more pending.  So please do go ahead and update
the translations asap.  Thanks.  -k

--- texlive-en.tex	(revision 40950)
+++ texlive-en.tex	(working copy)
@@ -1093,9 +1093,9 @@
 those in the \TeX\ trees.  They do these via related but not identical
 methods.
 
-On Windows, fonts shipped with \TL\ are made available to \XeTeX\
-automatically.  But if you have installed the \filename{xetex} package
-on a Unix-compatible system, you need to configure your system to be
+On Windows and \MacOSX, fonts shipped with \TL\ are made available to
+\XeTeX\ automatically.  But if you have installed the \filename{xetex}
+package on another Unix system, you need to configure your system to be
 able to find the fonts shipped with \TL\ via system name lookup, and not
 just filename lookup.
 
@@ -3553,8 +3553,16 @@
 
 MetaPost: Bug fixes and internal preparations for MetaPost 2.0.
 
-pdf\TeX: Use environment variable \code{SOURCE\_DATE\_EPOCH} (if set)
-for timestamps; new primitives \cs{pdfinfoomitdate}, \cs{pdftrailerid},
+\code{SOURCE\_DATE\_EPOCH} support in all engines except Lua\TeX\ (which
+will come in the next release) and original \code{tex} (intentionally
+omitted): if the environment variable \code{SOURCE\_DATE\_EPOCH} is set,
+its value is used for timestamps in the PDF output.  If
+\code{SOURCE\_DATE\_EPOCH\_TEX\_PRIMITIVES} is also set, the
+\code{SOURCE\_DATE\_EPOCH} value is used to initialize the \TeX\
+primitives \cs{year}, \cs{month}, \cs{day}, \cs{time}.  The pdf\TeX\
+manual has examples and details.
+
+pdf\TeX: new primitives \cs{pdfinfoomitdate}, \cs{pdftrailerid},
 \cs{pdfsuppressptexinfo}, to control values appearing in the output
 which normally change with each run.  These features are for PDF output
 only, not DVI.