llama : clarify comment about pp and tg graphs [no ci] (#14895)

llama : clarify comment about pp and tg graphs [no ci] (#14895)

* llama : clarify comment about pp and tg graphs [no ci]

This commit clarifies the comment in `llama-context.cpp` regarding the
prefill prompt (pp), and token generation (tg) graphs.

The motivation for this is that I've struggled to remember these and had
to look them up more than once, so I thought it would be helpful to add
a comment that makes it clear what these stand for.

* squash! llama : clarify comment about pp and tg graphs [no ci]

Change "pp" to "prompt processing".