description of how reads/writes work in detail

Adding detailed descriptions of the following to the md(4) manpage:
* How reads/writes work in detail, especially with respect to the
  minimum/maximum number of bytes that are always fully read/written.
* How reads/writes work when the array is degraded and or when a rebuild takes
  place.
* Some general concepts of how the chunksize affect reads/writes.

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>