Document toggles

[albfan]

there're lot of undocumented toggles:

let s:ag.toggle.literal_vsel = 1
let s:ag.toggle.case_ignore = 0
let s:ag.toggle.hidden = 0
let s:ag.toggle.unrestricted = 0
let s:ag.toggle.skip_vcs_ignore = 0
let s:ag.toggle.case_smart = 1
let s:ag.toggle.collapse_results = 0
let s:ag.toggle.goto_exact_line = 0
let s:ag.toggle.open_first = 0
let s:ag.toggle.ui_text = 0
let s:ag.toggle.code_variants = 0
let s:ag.toggle.debug = 0
let s:ag.toggle.highlight = 0