Improve documentation for lazy parameters

[dlangBugzillaToGithub]

Alexey Kulentsov (@crimaniak) reported this on 2017-11-12T16:58:21Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=17979

Description

Lazy parameters in fact implemented using delegates and affect @nogc possibility. I propose additions to documentation:

https://dlang.org/spec/function.html#parameters section

It needs to write that the delegate is used, and explicitly document what exactly happens in the case of several occurrences of the used parameter - whether the delegate is called each time, or only the first, and then the returned value is used (affects non-pure lazy parameters).

https://dlang.org/spec/garbage.html#op_involving_gc

It will be useful to mention lazy parameters in this list too.