Added INCLUDE_PREFIX option for flatbuffers_generate_headers (#5866)

Added INCLUDE_PREFIX option for flatbuffers_generate_headers (#5866)

This option was in an earlier revision of flatbuffers_generate_headers
but was lost while refactoring by mistake. We have it as a function
argument rather than one of the optional flags so that we can use it to
inform the rule where to place the generated headers, so that the file
location matches the prefix that will be added to the filenames in the
generated files.

Also fixed some of the documentation in the file.