man: "the initial RAM disk" → "the initrd"

man: "the initial RAM disk" → "the initrd"

In many places we spelled out the phrase behind "initrd" in full, but this
isn't terribly useful. In fact, no "RAM disk" is used, so emphasizing this
is just confusing to the reader. Let's just say "initrd" everywhere, people
understand what this refers to, and that it's in fact an initramfs image.

Also, s/i.e./e.g./ where appropriate.

Also, don't say "in RAM", when in fact it's virtual memory, whose pages
may or may not be loaded in page frames in RAM, and we have no control over
this.

Also, add <filename></filename> and other minor cleanups.