Explicitly note that :consistent includes no-ub (#49693)

* Explicitly note that :consistent includes no-ub

We may split these effects in the future, but currently :consistent-cy
requires the absence for undefined behavior for the function. There
were a few questions about this, so explicitly document this in the
help text.

* Update base/expr.jl

Co-authored-by: Sukera <11753998+Seelengrab@users.noreply.github.com>

---------

Co-authored-by: Shuhei Kadowaki <40514306+aviatesk@users.noreply.github.com>
Co-authored-by: Sukera <11753998+Seelengrab@users.noreply.github.com>

base/expr.jl

@@ -513,6 +513,13 @@ The `:consistent` setting asserts that for egal (`===`) inputs:
     even for the same world age (e.g. because one ran in the interpreter, while
     the other was optimized).
 
+!!! note
+    The `:consistent`-cy assertion currrently includes the assertion that the function
+    will not execute any undefined behavior (for any input). Note that undefined behavior
+    may technically cause the function to violate other effect assertions (such as
+    `:nothrow` or `:effect_free`) as well, but we do not model this, and all effects
+    except `:consistent` assume the absence of undefined behavior.
+
 !!! note
     If `:consistent` functions terminate by throwing an exception, that exception
     itself is not required to meet the egality requirement specified above.