Correct docs to reflect new log_level names in post-tracing Rocket

[mcclure]

In 0.5.1, the log levels are "off", "normal", "debug", and "critical". In current master, due to adoption of the tracing crate, they are "off", "error", "warn", "info", "debug", "trace", or 0-5. This is not currently reflected in the documentation. (Which is bad, because if you check out master and follow the documentation currently in the docs/ folder, you will set your log_level to "normal" or "critical", and the program will fail to boot.) Patch addresses this.

Quirks of patch:

• After switching to tracing, it appears the numbers 0-5 are also allowed as values for log_level. I don't know what strings these correspond to, so I didn't try to document them. I will fix this if you ask.
• I changed one line to fix an ambiguity caused by the fact info and debug are now separate log levels.

[martynp]

Thank you for the patch - I had already addressed most of these issues on my fork (rustfoo/rkt@a6aaece - went in a while ago, but my git foo is bad this morning I re-wrote the history fixing a typo)

But you have highlighted a few areas that I missed. Your patch is not directly applicable to my fork as I have moved a lot of things around - also the banner blocks have changed significantly - I have created a new PR incorporating those fixes here rustfoo/rkt#20.

[mcclure]

All right— in case we are eventually able to get hold of the original Rocket maintainer, are there additional documentation fixes from your fork I should try to port back into this PR?