Minor documentation nits

[mlawren]

A couple of things look funny to me:

1. If you understand the same convention around square brackets that I do, then the synopsis indicates to me that an argument is optional, when in fact one is required. Ideally just this?:

   rfc <command> | <number>

2. If angle brackets above indicate user-provided argument then for consistency I would expect the same for sub-commands:

   rfc search <query>

3. Along the same lines, if you don't bold the sync synopsis keywords, why do you underline the search synopsis keyword? Or vice-versa ...

4. The CURL environment variable description is not quite a sentence. Perhaps "Force the use of curl (default) or wget to retrieve RFC files"?

[bfontaine]

1. I think this was because you can run rfc without argument, although it’s useless. I fixed it.
2. I removed the brackets in 1.
3. the arguments of the sync command are fixed words, while the one for search is a placeholder. In the source I used <query> for the placeholder and `week` or `month` for the fixed words. Would you expect all arguments to be underlined (or bold), even if they are of different “type”?
4. Fixed.