Robert Haas shares some thoughts:
In my opinion, the PostgreSQL documentation is simultaneously excellent and fairly poor, and both its excellence and its shortcomings are direct results of the process by which the documentation is produced. The PostgreSQL documentation is stored in the same git repository as the source code, and anyone who patches the source code so as to change documented behavior must also patch the documentation to match.
This means that nearly all documentation updates are made by the developer who is most familiar with what is changing in the code, or sometimes by another developer who has studied those changes closely. Therefore, the documentation is usually extremely accurate. Sure, there are oversights, but it would be incredible to discover that some PostgreSQL command has a documented option which doesn’t actually exist, or that a parameter which is documented to take a string argument actually takes an integer or a Boolean. Typically, the descriptions of what SQL statements do and how that behavior is changed by parameter settings or options passed to the command itself are crisp and precise.
But read the whole thing, as there are downsides to this approach.