Minor documentation updates for consistency and clarity

[sadielbartholomew]

Quick one to make some minor formatting updates, mostly for the command-line help descriptions (I went in to update the documentation README and thought I should include these updates), namely:

• instead of listing the documentation build requirements in the README, pointing to the new docs-requirements.txt file created in #60 so we don't have two equivalent lists to keep in sync;
• being more consistent in the command-line descriptions by, e.g., using only single quotes around all keywords (means the generated CLI reference emphasises them via italics but avoiding double quotes which will clutter the terminal --help output) and listing defaults at the end;
• adding default information for --jobinfo which was missing before, to be explicit and clear;
• capitalising the first word from one docstring which was inconsistent with the others coming through in the API reference.

[sadielbartholomew]

(Pretty trivial PR to review, so open to anyone as reviewer and won't take long. Thanks!)