Minor fixes to mdrun performance documentation

Minor fixes to mdrun performance documentation

In "Examples for mdrun on one node," third example description, the respective number
of thread-mpi ranks and OpenMP threads per rank were reversed.

In "Examples for mdrun on one node," 6th example. For 12 logical cores, the pinoffsets
should be 0 and 6, respectively (I think)

A few command line examples of running mdrun with more than 1 node used gmx rather
than gmx_mpi

Several spelling/grammar/tense error/linking issues addressed.

Change-Id: I014bc52d55cda1cbd05843cb8e960c2a2d7cbb47