Update description

README.md

@@ -1010,10 +1010,13 @@ export interface TTFBAttribution {
    */
   requestDuration: number;
   /**
-   * The total time from the first byte of the response was received until the
-   * first byte of the document was received. This will only be non-zero for
-   * servers using Early Hints and where browsers support sending this additional
-   * timing. This time is after the TTFB metric.value.
+   * The total time, after TTFB, after which the document started to be
+   * received (i.e. when the document HTTP headers start to artive).
+   * This will only be non-zero for servers using Early Hints and where
+   * browsers support sending this additional timing. It is the time between
+   * the Early Hints first response (TTFB) and when the actual document
+   * response started and is useful to understand how much later the document
+   * TTFB is from the actual TTFB.
    */
   documentDuration: number;
   /**

src/types/ttfb.ts

@@ -63,10 +63,13 @@ export interface TTFBAttribution {
    */
   requestDuration: number;
   /**
-   * The total time from the first byte of the response was received until the
-   * first byte of the document was received. This will only be non-zero for
-   * servers using Early Hints and where browsers support sending this additional
-   * timing. This time is after the TTFB metric.value.
+   * The total time, after TTFB, after which the document started to be
+   * received (i.e. when the document HTTP headers start to artive).
+   * This will only be non-zero for servers using Early Hints and where
+   * browsers support sending this additional timing. It is the time between
+   * the Early Hints first response (TTFB) and when the actual document
+   * response started and is useful to understand how much later the document
+   * TTFB is from the actual TTFB.
    */
   documentDuration: number;
   /**