WolframLang: Show Function Help

Type ?FunctionName in Notebook .

For example, if you want to see the doc for the function StringLength:

(* return help string of StringLength *)
(* FullForm syntax *)
Information[ "StringLength" ]

(* shortcut syntax  *)
?StringLength

If you are runnig WolframScript in terminal, use:

TextForm[ ?StringLength ]

(* 
prints

StringLength["string"] gives the count of characters in a string.

 *)