Issue #24531: Document that encoding lines cannot follow non-comment lines.

Patch from Terry Reedy

Doc/reference/lexical_analysis.rst

@@ -109,7 +109,9 @@ Encoding declarations
 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
 encoding declaration; the first group of this expression names the encoding of
-the source code file. The recommended forms of this expression are ::
+the source code file. The encoding declaration must appear on a line of its
+own. If it is the second line, the first line must also be a comment-only line.
+The recommended forms of an encoding expression are ::
 
    # -*- coding: <encoding-name> -*-
 
@@ -126,8 +128,7 @@ If an encoding is declared, the encoding name must be recognized by Python. The
 encoding is used for all lexical analysis, in particular to find the end of a
 string, and to interpret the contents of Unicode literals. String literals are
 converted to Unicode for syntactical analysis, then converted back to their
-original encoding before interpretation starts. The encoding declaration must
-appear on a line of its own.
+original encoding before interpretation starts.
 
 .. XXX there should be a list of supported encodings.