omero: Cpp: Document FindIce changes

- Document FindIce cmake options
- Replace SLICEPATH with ICE_SLICE_DIR
- Remove documentation obsoleted by Ice autodetection
omero: Cpp: Document SLICEPATH workaround
omero: Cpp: Correct cmake link
omero: Cpp: Mark up cmake as a program
omero: Cpp: Reword version restrictions
omero: Cpp: Remove ICE_HOME from examples
omero: Cpp: Remove double space after period
Merge pull request #938 from rleigh-dundee/cmake-ice

omero: Cpp: Document FindIce changes