bandwidth-file: Add HeaderLine non-terminal

To make more clear that header lines do not contain spaces and simplify
parsers.
This does not change the format.

Closes: #30311

bandwidth-file-spec.txt

@@ -112,6 +112,7 @@
 
     Line ::= ArgumentChar* NL
     RelayLine ::= KeyValue (SP KeyValue)* NL
+    HeaderLine ::= KeyValue NL
     KeyValue ::= Keyword "=" Value
     Value ::= ArgumentCharValue+
     ArgumentCharValue ::= any printing ASCII character except NL and SP.
@@ -141,6 +142,14 @@
 
 2.2. Header List format
 
+  It consists of a Timestamp line and zero or more HeaderLines.
+
+  All the header lines MUST conform to the HeaderLine format, except
+  the first Timestamp line.
+
+  The Timestamp line is not a HeaderLine to keep compatibility with
+  the legacy Bandwidth File format.
+
   Some header Lines MUST appear in specific positions, as documented
   below. All other Lines can appear in any order.