IJG dox: Wordsmithing and formatting tweaks - Remove the section in libjpeg.txt that advised against building libjpeg as a shared library. We obviously do not follow that advice, and libjpeg-turbo does guarantee backward ABI compatibility in our libjpeg API library, even though libjpeg did not and does not. (Future expansion of our libjpeg API library, if necessary, will be accomplished using get/set functions that store the new parameters in the opaque master structs. Refer to https://github.com/mozilla/mozjpeg/commit/db2986c96fcebe10c8700081578e5305fd7bc7dc.) - Unmention install.txt, which was never relevant to libjpeg-turbo and was removed in v1.3 (6f96153c67e9a88775b457f998fc7fdd64c62cee). - Remove extraneous spaces. - Document the fact that TWO_FILE_COMMANDLINE must be defined in order to use the two-file interface with cjpeg, djpeg, jpegtran, and wrjpgcom. libjpeg-turbo never enables that interface by default.