Update buffile.h/.c comments for removal of non-temp option.

Commit 11e264517 removed BufFile's isTemp flag, thereby eliminating
the possibility of resurrecting BufFileCreate().  But it left that
function in place, as well as a bunch of comments describing how things
worked for the non-temp-file case.  At best, that's now a source of
confusion.  So remove the long-since-commented-out function and change
relevant comments.

I (tgl) wanted to rename BufFileCreateTemp() to BufFileCreate(), but
that seems not to be the consensus position, so leave it as-is.

In passing, fix commit f0828b2fc's failure to update BufFileSeek's
comment to match the change of its argument type from long to off_t.
(I think that might actually have been intentional at the time, but
now that 64-bit off_t is nearly universal, it looks anachronistic.)

Thomas Munro and Tom Lane

Discussion: https://postgr.es/m/E1eFVyl-0008J1-RO@gemulon.postgresql.org

src/include/storage/buffile.h

@@ -1,7 +1,7 @@
 /*-------------------------------------------------------------------------
  *
  * buffile.h
- *	  Management of large buffered files, primarily temporary files.
+ *	  Management of large buffered temporary files.
  *
  * The BufFile routines provide a partial replacement for stdio atop
  * virtual file descriptors managed by fd.c.  Currently they only support