#401 lzma-specification improvements

[Andrew Bainbridge]

I've just read through lzma-specification.txt and made tweaks as I went. The changes are mostly as follows:

• Added and removed lots of "a" and "the". Lots of other small grammar tweaks and readability improvements.
• Made formatting more consistent and removed trailing whitespace.
• Tried to consistently say "sliding window" instead of just "window".
• Capitalized lzma and xz in several places.
• Replaced "code" with "implementation" in a couple of places where I felt "code" could wrongly be taken to mean the encodings in the LZMA stream.

I tried not to make any stylistic changes and tried even harder not to change the technical content.