Use distinct savelowerto/redirectlowerto file names

[muzimuzhi]

This PR makes the changes I suggested in 4918308#r128530467

Before, file name suffix _bspsave is used once in savelowerto doc example, and suffix _bspsave2 is used twice in savedelimiter doc examples.

Although using the same suffix is OK since each of the new files is input right after creation, using different suffixes in doc examples for different options and naming them consistently with a mono-increasing number suffix (like _bspsave, _bspsave2, _bspsave3, and maybe _bspsave4) is not a bad idea. (Sticking to the same suffix is also not bad.)

Update: Using distinct temp file names has the benefit of checking the verbatim-writing is working.

[T-F-S]

Thank you for thorough reading. I will do as suggested.