seq_buf: Fix kernel documentation

seq_buf: Fix kernel documentation

There are plenty of issues with the kernel documentation here:
  - misspelled word "sequence"
  - different style of returned value descriptions
  - missed Return sections
  - unaligned style of ASCII / NUL-terminated / etc
  - wrong function references

Fix all these.

Link: https://lkml.kernel.org/r/[email protected]
Cc: Andrew Morton <[email protected]>
Signed-off-by: Andy Shevchenko <[email protected]>
Reviewed-by: Randy Dunlap <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>