Fix child process docs

Fix a number of outright errors in child process documentation, as well as improve the general readability of it by using hyperlinks, documenting defaults, ordering the docs consistently, etc.
Also, the whole paragraph is basically just a factoring out (not by me) of common prose for options 1, 3, 4, and 5. Without this info, you don't know what the pipes create in 1., for example, look like in the child, or how to use them.