Fix +RTS -Z flag documentation

Stack squeezing is done on context switch, not on GC or stack overflow.
Fix the documentation.

Fixes #17685

[ci skip]

docs/users_guide/runtime_control.rst

@@ -1332,7 +1332,7 @@ recommended for everyday use!
 
 .. rts-flag:: -Z
 
-    Turn *off* "update-frame squeezing" at garbage-collection time.
+    Turn *off* update frame squeezing on context switch.
     (There's no particularly good reason to turn it off, except to
     ensure the accuracy of certain data collected regarding thunk entry
     counts.)

rts/RtsFlags.c

@@ -324,7 +324,7 @@ usage_text[] = {
 "  -S[<file>] Detailed GC statistics (if <file> omitted, uses stderr)",
 "",
 "",
-"  -Z         Don't squeeze out update frames on stack overflow",
+"  -Z         Don't squeeze out update frames on context switch",
 "  -B         Sound the bell at the start of each garbage collection",
 #if defined(PROFILING)
 "",