Reporting bugs effectively is a fundamental way to help PostGIS
development. The most effective bug report is that enabling PostGIS
developers to reproduce it, so it would ideally contain a script
triggering it and every information regarding the environment in which it
was detected. Good enough info can be extracted running
postgis_full_version() [for postgis] and
version() [for postgresql].
If you aren't using the latest release, it's worth taking a look at its release changelog first, to find out if your bug has already been fixed.
Using the PostGIS bug tracker will ensure your reports are not discarded, and will keep you informed on its handling process. Before reporting a new bug please query the database to see if it is a known one, and if it is please add any new information you have about it.
You might want to read Simon Tatham's paper about How to Report Bugs Effectively before filing a new report.
The documentation should accurately reflect the features and behavior of the software. If it doesn't, it could be because of a software bug or because the documentation is in error or deficient.
Documentation issues can also be reported to the PostGIS bug tracker.
If your revision is trivial, just describe it in a new bug tracker issue, being specific about its location in the documentation.
If your changes are more extensive, a Subversion patch is definitely preferred. This is a four step process on Unix (assuming you already have Subversion installed):
Check out a copy of PostGIS' Subversion trunk. On Unix, type:
svn checkout http://svn.osgeo.org/postgis/trunk/
This will be stored in the directory ./trunk
Make your changes to the documentation with your favorite text editor. On Unix, type (for example):
Note that the documentation is written in SGML rather than HTML, so if you are not familiar with it please follow the example of the rest of the documentation.
Make a patch file containing the differences from the master copy of the documentation. On Unix, type:
svn diff trunk/doc/postgis.xml > doc.patch
Attach the patch to a new issue in bug tracker.