Documentation: geospatial PDF generation

[rouault]

This ticket is to track the new feature introduced in #4216 that likely needs some documentation. #4216 also contains a sample mapfile and dataset.

A start for the documentation is the comments of msTransformToGeospatialPDF() :

/* msSaveImagePostPDFProcessing() will call the GDAL PDF driver to add geospatial */
/* information to the regular PDF generated by cairo. This is only triggered if the */
/* GEO_ENCODING outputformat option is set (to ISO32000 or OGC_BP). Additionnal */
/* options can be provided by specifying outputformat options starting with */
/* METADATA_ITEM: prefix. For example METADATA_ITEM:PRODUCER=MapServer */
/* Those options are AUTHOR, CREATOR, CREATION_DATE, KEYWORDS, PRODUCER, SUBJECT, TITLE */
/* See http://gdal.org/frmt_pdf.html documentation. */

[havatv]

Documented in mapfile-> outputformat in r13199. Closing.