Reflow all the language stack `README-content.md` files to 80 columns and incorporate Fred's changes

This also includes Fred's updates to the `README-short.txt` files, and a bunch of very minor changes for consistency (using "PHP" instead of "php", ending sentences that describe and lead into a code block with a ":", using "Go" instead of "Golang" and "Hy" instead of "Hylang" within prose, not using periods at the end of headlines, etc).

A really fun one included here is that Java mentioned both using GNU Make inside the container (and `java` doesn't include `make` presently), and some of the prose mentioned `go build` after we ran `javac`, which was extra neat.

I've also reflowed the footer here, since it looks like Fred's got his reflow set at 90 columns, not 80 (which is half the reason I made this PR, the other half being that I wanted to go over them all one more time myself to fix more minor inconsistencies).

cc @fredlf (would you mind taking one last gander over these to make sure I haven't missed anything that sticks out to you on a last read-over?)

Closes #7
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(To make reviewing this simple, I haven't run `./update.sh` yet - once it's been reviewed and we're all happy with it and thus have merged it, I'll run `./update.sh` to update all the `README.md` files too.)