Docs are missing clear pointers to `inproc` and similar

[Rebecca Turner]

When translating Bash scripts, one of the most common tasks is to run a command and receive its output (or check its exit status). Turtle provides helpers for these (like inproc and procs), but the docs don't show them prominently so (despite looking at the docs several times before) I had no clue they existed until @Gabriella439 told me so directly!