Add <code> tags to Player extern docs.
Add <code> tags to Player extern docs.

This also adds a mentions the 'largegap' event.

Fixes #2097

Change-Id: I2ecf9f40a12f16741c1a01a455e70f3aaac62a5c

shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,tvoli/shaka-player