Updates Plugin Configuration 

Updates the plugin configuration section with information on the `ignoreMissing` option.
Spelling error in plugins.rst

Corrected boostrap to bootstrap
Add some details around form tampering.

Refs cakephp/cakephp#1967
Merge pull request #1139 from jtyost2/patch-1

Updates Plugin Configuration with ignoreMissing option
Fix formatting.

Refs cakephp/docs#1139