[patch, fortran] ATAN2 and -fsign-zero doc fixes

[FX]

Hi all,

Attached patch fixes the documentation of ATAN2 regarding signed zeros (aka 
PR49336) and the documentation of option -fsign-zero regarding I/O behaviour 
(aka PR49188). While I was at it, I also removed some lonely contractions 
(it's, 'll, can't, don't, etc.)

Tested with "make man info html pdf", OK to commit to trunk?

Cheers,
FX

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