Suggested update to the FPGA Synthesis sub-section of the User Manual

The User Manual has a short FPGA Synthesis sub-section. Based on recent discussions (see #943), we should consider updating this text:

1. The sentence FPGA synthesis is only supported for CV32E40P is confusing. Maybe the word "only" is not intended to be there?
2. We could list the FPGA synthesis tools that have been successfully used for this core.
3. Mention that Quartus Prime Standard is not supported (Quartus Prime Pro is required).