Where can I find documentation on SQL Server internals?
I'm looking for docs on the internal format of the WAL log
  


The SQL Server Log File is not publicly documented, but there is a partner program for 3rd parties that want to build solutions that use it directly.  EG Attunity, Quest, etc.


  
    And, moreover, is there anyway to know what exactly is documented with MSDN without first paying for it?
  


There is ZERO additional documentation available to MSDN subscribers.  All published documentation and support material is published to the web.

The best sources for SQL Server Internals are the work of Kalen Delaney https://www.red-gate.com/simple-talk/author/kalen-delaney/ and Paul Randal https://www.sqlskills.com/blogs/paul/.

In particular Paul has some posts on reading the SQL Log. eg
https://www.sqlskills.com/blogs/paul/using-fn_dblog-fn_dump_dblog-and-restoring-with-stopbeforemark-to-an-lsn/