documentation change for pressure variable

The XDC documentation for the pressure variable is incomplete- it reads
"pressure" rather than "pressure reduced to sea-level".  It has always
been sea-level pressure although the provider of the data to the XDC (NCDC)
documents it incorrectly as surface pressure.

• ebeam_el_stn
• site_num_stn
• vbeam_el_stn
• wspd
• rh
• nbeam_az_stn
• temp
• press
• alt_stn
• anem_ind
• alt
• time_offset
• orient_n_stn
• rain
• checkout_mode
• nbeam_el_stn
• ebeam_az_stn
• lon
• dp
• vbeam_az_stn
• wdir
• avg_count
• lat
• lon_stn
• base_time
• lat_stn
• magnetic_dcn_stn