Improve documentation with a motivation section

docs/00-intro.md

@@ -14,6 +14,18 @@ $field[1]->value()->toString(); // returns 'application/xhtml+xml'
 $field[1]->parameterByKey(key: 'q', default: 1.0); // returns 1.0 if the parameter is not defined
 ```
 
+## Motivation
+
+While they are plenty of HTTP headers and trailers, they have historically come each with their own set of 
+rules and constraints when it came to parsing and serializing them. Trying to use the parsing logic of a cookie header
+to parse an accept header will fail. The various parsing logics hinders HTTP headers usage, modernization or security.
+The [Structured Field RFC](https://www.rfc-editor.org/rfc/rfc9651.html) aim at tackling those issues by unifying
+HTTP headers and trailers.
+
+New HTTP headers or trailers (called fields) are encouraged to use the new RFC algorithm and ongoing discussions are
+happening to [retrofit existing headers that do not match the RFC](https://httpwg.org/http-extensions/draft-ietf-httpbis-retrofit.html) into new 
+shapes that would be compatible with it.
+
 ## Foreword
 
 > [!CAUTION]

docs/04-item.md

@@ -4,7 +4,7 @@ This is the structure from which you will be able to access the actual field con
 
 ## Items value
 
-The height (8) defined value types are all attached to an `Item` object where their value and
+The eight (8) defined value types are all attached to an `Item` object where their value and
 type are accessible using the following methods:
 
 ```php