Document signals (SIGSTOP, SIGVTALRM, SIGPROF) to fix sphinx references

[LamentXU123]

After #139843 I've found more undocumented signals that will cause reference warnings.

C:\Users\admin\Desktop\cpython-main\Doc\library\signal.rst:475: WARNING: py:data reference target not found: SIGSTOP [ref.data]
C:\Users\admin\Desktop\cpython-main\Doc\library\signal.rst:496: WARNING: py:const reference target not found: SIGVTALRM [ref.const]
C:\Users\admin\Desktop\cpython-main\Doc\library\signal.rst:496: WARNING: py:const reference target not found: SIGPROF [ref.const]

similar to the farmer one I'm adding those documents to signal.rst, with description in https://pubs.opengroup.org/onlinepubs/009695399/basedefs/signal.h.html . The document is ordered in the same way.

Linked PRs

• gh-139843: Document signals (SIGSTOP, SIGVTALRM, SIGPROF) to fix sphinx references #139896

[picnixz]

Let's reuse #139843 instead.