Clarify that class Ascii is from Guava.

This avoids initial confusion for stupid users (like me!) who when working outside of Google internal codebase and when first encountering this error go look for class Ascii in the JDK.

Author: vorburger

core/src/main/java/com/google/errorprone/bugpatterns/StringCaseLocaleUsage.java

@@ -86,7 +86,7 @@ private static SuggestedFix suggestAscii(MethodInvocationTree tree, VisitorState
     var fix =
         SuggestedFix.builder()
             .setShortDescription(
-                "Replace with Ascii.toLower/UpperCase; this changes behaviour for non-ASCII"
+                "Replace with Guava's Ascii.toLower/UpperCase; this changes behaviour for non-ASCII"
                     + " Strings");
     String ascii = SuggestedFixes.qualifyType(state, fix, "com.google.common.base.Ascii");
     fix.replace(

docs/bugpattern/StringCaseLocaleUsage.md

@@ -10,4 +10,5 @@ If this kind of regionalisation is desired, use
 `.toLowerCase(Locale.getDefault())` to make that explicit. If not,
 `.toLowerCase(Locale.ROOT)` or `.toLowerCase(Locale.ENGLISH)` will give you
 casing independent of the user's current `Locale`. If you know that you're
-operating on ASCII, prefer `Ascii.toLower/UpperCase` to make that explicit.
+operating on ASCII, prefer `Ascii.toLower/UpperCase` (from Guava, if available)
+to make that explicit.