The following changes take effect:
1. Annotate FixedStringBuffer => `FixedStringBuffer` in many places.
2. Remove non-existing helpers for FixedStringBuffers. I added them
previously but even then they were removed in a following fixup so these
references were never valid. Therefore let's just put a vague reference
to the fact that we have some helpers for this class in the Kernel, and
let people to figure out quickly by themselves for this topic.
3. Put a sentence to explain that FixedStringBuffer objects are not only
being used in syscall handling code, but also for storing actual data in
both the Thread and Process classes as well.
