Improve documentation to explain that Inverted Sections uses falsy

[Lukenickerson]

The documentation (here and here) explains:

The block of an inverted section is rendered only if the value of that section's tag is null, undefined, false, or an empty list.

they will be rendered if the key doesn't exist, is false, or is an empty list.

But the spec says

If the data is not of a list type, it is coerced into a list as follows: if the data is truthy (e.g. !!data == true), use a single-element list containing the data, otherwise use an empty list.

(From issue #186)

The documentation should be more clear that the inverted section uses falsy. Personally I was confused when a value of 0 was triggering the inverted section (I read the docs thinking "is" is like ===).

[dasilvacontin]

If no one offers to send a PR, I'll assign this to myself.

[phillipj]

Fixed with 5f06df2