Documentation: String (#27)

* initial comments

* update

* some fixes

* add copyright comment

* updates

* update module comment

Co-authored-by: Gabriel Nordeborn <gabbe.nord@gmail.com>

* some fixes

* update

* update

* update

* update

* update

* some fixes

* Update src/Core__String.resi

Co-authored-by: Gabriel Nordeborn <gabbe.nord@gmail.com>

* fix indentation

* fix float

* update CHANGELOG.md

---------

Co-authored-by: Gabriel Nordeborn <gabbe.nord@gmail.com>

Author: aspeddro

CHANGELOG.md

@@ -36,3 +36,4 @@
 - Docstrings for `RegExp`. https://github.com/rescript-association/rescript-core/pull/47
 - Docstrings for `Date`. https://github.com/rescript-association/rescript-core/pull/61
 - Docstrings for `Float`. https://github.com/rescript-association/rescript-core/pull/54
+- Docstrings for `String`. https://github.com/rescript-association/rescript-core/pull/27