fill in missing documentation

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@19520 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2409 Speed up reading large files by 
 * Removing random permutation element reads from ParMETIS partitioning and reading contiguously where possible
 * Spotting contiguous reading patterns in binary readers and avoiding an expensive seekg()

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@19522 b5f69653-2bf5-0310-8201-b0dafb8e1405