UPSTREAM: signal: improve comments

Improve the comments for pidfd_send_signal().
First, the comment still referred to a file descriptor for a process as a
"task file descriptor" which stems from way back at the beginning of the
discussion. Replace this with "pidfd" for consistency.
Second, the wording for the explanation of the arguments to the syscall
was a bit inconsistent, e.g. some used the past tense some used present
tense. Make the wording more consistent.

Signed-off-by: Christian Brauner <christian@brauner.io>

(cherry picked from commit c732327f04a3818f35fa97d07b1d64d31b691d78)

Bug: 135608568
Test: test program using syscall(__NR_sys_pidfd_open,..) and poll()
Change-Id: Iaa289e1bc2f96366102e51241accbb97099553d4
Signed-off-by: Suren Baghdasaryan <surenb@google.com>