Revise gmt5syntax.in and add documentation

[PaulWessel]

This perl script which is configured by CMake and ends up as the program gmt5syntax in share/tools can be used to translate old GMT4 scripts to modern syntax (basically, prepending gmt to commands missing it). There seems to be a couple of issues related to this script:

1. The documentation on Migration has no mention of it yet it seems like a great solution to help people convert to GMT6 classic syntax.
2. The name (gmt5...) is unfortunate.

I suggest the following changes:

1. Rename the script to something without gmt5 in it, e.g., convertgmt4, others?)
2. Add a paragraph on this in the Migration section.

We should probably also give it a shot and maybe even show its effect on a small GMT4 script in that section.