Fix documentation for experimental_per_crate_rustc_flag. (#3380)

This setting was introduced in [this
commit](6571cde),
and from the context there
([docs](6571cde#diff-2a806da393e47c07ffe67c78ace69eb488b4ac44b029a46d8237b8e2a05637be),
[tests](6571cde#diff-7cfe86db91ef77d2663a7f891492547fcae21fc081ae947532f031585eb86f1d)),
it seems clear that the intent is that the flag should apply to non-exec
configurations.

When [this
commit](eb1fe7d#diff-d516c822cec75e56bb5c193fbd7c7dfd28d8f982224175324da28b61bf6a1fe3)
later introduced the documentation being changed here, it seems that it
inadvertently misunderstood the flag as applying _only_ to exec
configurations, rather than _not_ to exec configurations.

Author: martinboehme

rust/settings/settings.bzl

@@ -322,7 +322,7 @@ def experimental_per_crate_rustc_flag():
     The expected flag format is prefix_filter@flag, where any crate with a label or execution path starting
     with the prefix filter will be built with the given flag. The label matching uses the canonical form of
     the label (i.e `//package:label_name`). The execution path is the relative path to your workspace directory
-    including the base name (including extension) of the crate root. This flag is only applied to the exec
+    including the base name (including extension) of the crate root. This flag is not applied to the exec
     configuration (proc-macros, cargo_build_script, etc). Multiple uses are accumulated.
     """
     _per_crate_rustc_flag(