blockMesh: Added printing of the block description to the '-help' output

blockMesh -help

Usage: blockMesh [OPTIONS]
options:
  -blockTopology    write block edges and centres as .obj files
  -case <dir>       specify alternate case directory, default is the cwd
  -dict <file>      specify alternative dictionary for the blockMesh description
  -noFunctionObjects
                    do not execute functionObjects
  -region <name>    specify alternative mesh region
  -srcDoc           display source code in browser
  -doc              display application documentation in browser
  -help             print the usage

Block description

  For a given block, the correspondence between the ordering of
  vertex labels and face labels is shown below.
  For vertex numbering in the sequence 0 to 7 (block, centre):
    faces 0 (f0) and 1 are left and right, respectively;
    faces 2 and 3 are bottom and top;
    and faces 4 and 5 are front the back:

           4 ---- 5
      f3   |\     |\   f5
      |    | 7 ---- 6   \
      |    0 |--- 1 |    \
      |     \|     \|    f4
      f2     3 ---- 2

            f0 ----- f1

Using: OpenFOAM-dev (see www.OpenFOAM.org)
Build: dev-dc59c633