The N900 has three main storage partitions. It has a few more but these three are the most important:
1) root, about 220 MB, where all the system packages and some other, small programs go
2) opt, about 2 GB, where the rest of the "installable packages" go, as well as most of your package configuration
3) MyDocs, about 27 GB, where your pictures, videos, music etc goes

Space can be reallocated between opt and MyDocs but root is fixed. And, as you can see, rather small, which means it can fill up quite easily.

docpurge is a package, registered to be activated at the end of every installation. There are a few more, catorise may be one of them.

The purpose of docpurge is to, you've guessed it, purge the docs In other words, to remove the documentation that comes bundled with some packages. This is because the default place where most applications put their docs is in /usr/share/doc, which is on a root partition (by the way, "optification" is a process of moving files out of root to opt). You can see the effects of docpurge if you look in /usr/share/doc. On my device, there is a single file, README, with the contents, "Purged by docpurge".

As far as I know, docpurge is installed by default, which means /usr/share/doc is kept empty at all times and yes, you may have to repeat the exercise several times.