fcntl.2: Actual pipe capacity may in practice be less than nominal capacity

fcntl.2: Actual pipe capacity may in practice be less than nominal capacity

The number of bytes that can be written to the pipe may be less
(sometimes substantially less) than the nominal capacity. This
was confirmed with some testing. For example, when writing
4097-byte blocks to a pipe with 65536 byte capacity, only 45066
bytes could be written (i.e., 20470 bytes less than 65536).

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>