#18628: clarify index entry for source file encoding declaration.

Patch by Sam Lucidi.

Doc/reference/lexical_analysis.rst

@@ -76,7 +76,7 @@ are ignored by the syntax; they are not tokens.
 Encoding declarations
 ---------------------
 
-.. index:: source character set, encodings
+.. index:: source character set, encoding declarations (source file)
 
 If a comment in the first or second line of the Python script matches the
 regular expression ``coding[=:]\s*([-\w.]+)``, this comment is processed as an