text/template: add more package docs

[thewhitetulip]

We can add more comments to text/template package so the readers who use godoc will see somewhat more detailed material in the documentation rather than the sparse documentation which is currently present.

[bradfitz]

Which parts of text/template you would like to see more documentation on?

[thewhitetulip]

small concise examples about every function, I'll post a list soon

[pyrossh]

+1. More documentation on using the template tag would be useful because people generally use one large template and insert stuff into it.

{{template "content" .Data}}

[thewhitetulip]

We can give examples for using the eq operator, for go1.6 considerably new documentation has been added like the range operator. I feel that the documentation for text/template should sound less like documentation documentation, meaning too much theoretical since a documentation is basically an official guide into the API of the language, it should be like Python's documentation, least amount of theory and all examples.