I don't think that's what he's talking about. He wants stuff like the PDF he mentioned that go into detail on how to optimize your game or application to best take advantage of the specific GPU. Stuff to do, stuff to avoid, things like that. Most other GPU designers have documentation like that available, for example:
Qualcomm: https://developer.qualcomm.com/develop/ ... ion-adreno
AMD and Nvidia of course have vast amounts of developer guides and even entire books available on their websites.
That's exactly it. Saying we support OpenGL ES version X, etc. is fine for people just getting started learning but for people trying to get the best performance out of a system more information is useful.
e.g. When Nvidia added support for vertex buffer objects to their desktop GPU drivers, they released an application note describing how to use them. It also warned that whilst they supported 16 and 32 bit index buffer values, as defined in the specification, 32 bit values were slower on their hardware at the time and so 16 bit values should be preferred. That "don't do this it works but is slow" type of information allows developers to avoid performance problems with particular chipsets that may only become apparent during final testing or, for small developers who can't afford lots of test hardware, after release when customers start complaining.
Here's an RPi specific issue:-
There has been some discussion on the forum of the optimum amount of memory to allocate to the GPU in the bootstrap configuration file.
Typically for desktop OpenGL implementations you have a relatively small amount of separate graphics memory (GPU memory) and a much larger amount of main memory (CPU memory). OpenGL will manage caching the most frequently used textures in GPU memory and copy textures from CPU memory as needed. To do this it needs to keep a copy of all textures in CPU memory - even if they are also in GPU memory.
For software rendering, all memory is CPU memory so a software implementation doesn't need to do this.
The RPi hardware is a half way house between these two as, although it's all on the same chip, memory is split between the CPU and the GPU during bootstrap. Knowing the OpenGL memory usage strategy will be important - if a copy of all textures is kept in CPU memory, allocating 128Mb to the GPU may not be the best strategy since you'll reduce the amount you'd have for the OS and any applications you are running and may allocate memory to the GPU that you could never use.