Improved documentation of map()

[surdina]

Improved documentation of map() functions. This closes #671.

• Documentation for map() now divided into two separate sections for map() and map_if()/map_at(); reference to the latter added under 'see also'
• Removed obvious example (preserving column names)
• Added documentation on map_depth()
• Loaded parameters from map_if() and map_at() in modify() documentation

[lionel-]

Looks great! This is much better.

[lionel-]

Thanks!