Improve documentation about using /vsistdout/ for writing to standard output

[peifer]

OGR's BNA, CSV, GeoJSON, GeoRSS, GML, GPX, KML and PGDump seem to be able to use /vsistdout/ as output file for writing to standard output. However, only drv_pgdump.html mentions /vsistdout/. Other relevant driver pages should also mention this option.

Before I forget: consistency of aliases between /dev/stdout and /vsistdout/ could be improved between OGR drivers. As Even R wrote today on gdal-dev: Yes we probably lack consistency in that area. I think the stdout case should be removed (should not affect many people as it was unadvertized feature), and only accept /dev/stdout and /vsistdout/ which are less hacky solutions.

[Even Rouault]

r23022 /trunk/ (21 files in 11 dirs): 
OGR drivers: mentions in the doc that BNA, CSV, GPX, KML, GeoRSS, GML, LIBKML,
GeoJSON, PGDump and shapefile (non exhaustive list...) support the VSI File API;
for the ones supporting outputing to standard output (all of the above except
shapefile), accept both /dev/stdout and /vsistdout/ as filenames; remove
'stdout' as a valid alias that could be used in some of them (#4225, #4226)