Fixed some spelling and grammar issues (mostly inject doc)

Most changes are on the inject documentation, however some fixes are
for all occurrences of mistakes.
* Changes occurrences of "Lets" to "Let's" where appropriate
* Changed occurrences of "ajax" to "AJAX" where appropriate
* Changed occurrences of "jquery" to "jQuery"
* it can be trigger upon -> triggered
* [it is] ideal to understand -> ideal for understanding