Show a link to the doc when user passes an unsupported input type

Ported from 7eec01cdd6

CHANGELOG.md

@@ -21,3 +21,4 @@ HEAD
 * Remove `JsonVariant::shallowCopy()`
 * `JsonDocument`'s capacity grows as needed, no need to pass it to the constructor anymore
 * `JsonDocument`'s allocator is not monotonic anymore, removed values get recycled
+* Show a link to the documentation when user passes an unsupported input type

src/ArduinoJson/Deserialization/Reader.hpp

@@ -18,7 +18,9 @@ struct Reader {
   Reader(TSource& source) : source_(&source) {}
 
   int read() {
-    return source_->read();  // Error here? You passed an unsupported input type
+    // clang-format off
+    return source_->read();  // Error here? See https://arduinojson.org/v6/invalid-input/
+    // clang-format on
   }
 
   size_t readBytes(char* buffer, size_t length) {