be more assertive about what documents define

Author: robUx4

index_codec.md

@@ -50,7 +50,7 @@ It stores interleaved and timestamped audiovisual data using various codecs.
 To interpret the codec data, a mapping between the way the data is stored in Matroska and
 how it is understood by such a codec is necessary.
 
-This document intends to define this mapping for many commonly used codecs in Matroska.
+This document defines this mapping for many commonly used codecs in Matroska.
 
 # Notation and Conventions
 

index_tags.md

@@ -51,7 +51,7 @@ in a Matroska `Segment`. It can tag a whole `Segment`, separate `Tracks` element
 Some details about tagging are already present in [@!RFC9559, section 24].
 
 While the Matroska tagging framework allows anyone to create their own custom tags, it's important to have a common
-set of values for interoperability. This document intends to define a set of common tag names used in Matroska.
+set of values for interoperability. This document defines a set of common tag names used in Matroska.
 
 # Notation and Conventions