Fix documented Composer home variable name

Documentation fixes:

 - Fix documented Composer home variable name
 - Wrap long line
COMPOSER_HOME is set to `ENV[HOME]/.composer` if not existing, and the home var in config is set to the COMPOSER_HOME env var. It's not three types of home variables, just different levels (env var, default, and composer config). Anyway I guess it's ok simpler like this if I just merge, most people don't need to know or care about the internal home config.