[png-mng-misc] Revised PNG Extensions document and PNG Register of Chunks document

[Glenn Randers-P. <gl...@gm...>]

New versions of two PNG documents are available in the
https://ftp-osl.osuosl.org/pub/libpng/documents directory:

Register of public PNG chunks and keywords, Version 1.4.9
 https://ftp-osl.osuosl.org/pub/libpng/documents/pngreg-1.4.9-pdg.html

Extensions to the PNG 1.2 Specification, Version 1.4.1
 https://ftp-osl.osuosl.org/pub/libpng/documents/pngext-1.4.1-pdg.html
or
 https://ftp-osl.osuosl.org/pub/libpng/documents/pngextensions.html

I added a reference to RFC-2119 in the PNG Extensions) document.
Other than that, the content is unchanged, but the HTML markup
in both documents is revised to pass W3C validation tests.

Glenn