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

Author: LamentXU123

Doc/library/signal.rst

@@ -205,6 +205,12 @@ The variables defined in the :mod:`signal` module are:
 
    .. availability:: Unix.
 
+.. data:: SIGPROF
+
+   Profiling timer expired.
+
+   .. availability:: Unix.
+
 .. data:: SIGQUIT
 
    Terminal quit signal.
@@ -215,6 +221,10 @@ The variables defined in the :mod:`signal` module are:
 
    Segmentation fault: invalid memory reference.
 
+.. data:: SIGSTOP
+
+   Stop executing (cannot be caught or ignored).
+
 .. data:: SIGSTKFLT
 
    Stack fault on coprocessor. The Linux kernel does not raise this signal: it
@@ -243,6 +253,12 @@ The variables defined in the :mod:`signal` module are:
 
    .. availability:: Unix.
 
+.. data:: SIGVTALRM
+
+   Virtual timer expired.
+
+   .. availability:: Unix.
+
 .. data:: SIGWINCH
 
    Window resize signal.