Some fixes to the new docs

Hi
I'm a Groovy newbie, studying along the new docs. So I thought I'd fix typos and errors whenever I find them.
This is the first batch, up to §1.2.3
I've fixed an example on `!`, `&&` and `||` precedence (it wasn't proving anything, as written); I've added missing docs on `&&` short-circuiting; plus various typos.
It's all properly split into separate commits.
I've checked the output with `gradle asciidoctor` and ran `gradle testSingle*` on all test files I've touched.