[docs] Some rewording to the builtins

Plus some additional examples.

doc_src/begin.txt

@@ -9,9 +9,9 @@ begin; [COMMANDS...;] end
 
 `begin` is used to create a new block of code.
 
-The block is unconditionally executed. `begin; ...; end` is equivalent to `if true; ...; end`.
+A block allows the introduction of a new variable scope, redirection of the input or output of a set of commands as a group, or to specify precedence when using the conditional commands like `and`.
 
-`begin` is used to group a number of commands into a block. This allows the introduction of a new variable scope, redirection of the input or output of a set of commands as a group, or to specify precedence when using the conditional commands like `and`.
+The block is unconditionally executed. `begin; ...; end` is equivalent to `if true; ...; end`.
 
 `begin` does not change the current exit status itself. After the block has completed, `$status` will be set to the status returned by the most recent command.
 
@@ -23,6 +23,7 @@ The following code sets a number of variables inside of a block scope. Since the
 \fish
 begin
     set -l PIRATE Yarrr
+
     ...
 end