docs: rust: general-information: use real example

Currently the example in the documentation shows a version-based name
for the Kconfig example:

    RUSTC_VERSION_MIN_107900

The reason behind it was to possibly avoid repetition in case several
features used the same minimum.

However, we ended up preferring to give them a descriptive name for each
feature added even if that could lead to some repetition. In practice,
the repetition has not happened so far, and even if it does at some point,
it is not a big deal.

Thus replace the example in the documentation with one of our current
examples (after removing previous ones from the bump), to show how they
actually look like, and in case someone `grep`s for it.

In addition, it has the advantage that it shows the `RUSTC_HAS_*`
pattern we follow in `init/Kconfig`, similar to the C side.

Reviewed-by: Tamir Duberstein <tamird@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260405235309.418950-31-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Documentation/rust/general-information.rst

@@ -157,5 +157,5 @@ numerical comparisons, one may define a new Kconfig symbol:
 
 .. code-block:: kconfig
 
-	config RUSTC_VERSION_MIN_107900
-		def_bool RUSTC_VERSION >= 107900
+	config RUSTC_HAS_SPAN_FILE
+		def_bool RUSTC_VERSION >= 108800