docs: Add python block usage examples (#687)

* added some text

* updated with input/output syntax

Author: icweaver

docs/src/pythoncall.md

@@ -8,7 +8,7 @@ This package is in the general registry, so to install just type `]` in the Juli
 pkg> add PythonCall
 ```
 
-## Getting started
+## [Getting started](@id py_getting_started)
 
 Import the module with:
 
@@ -91,6 +91,52 @@ Python: ValueError('some error')
 With the functions introduced so far, you have access to the vast majority of Python's
 functionality.
 
+## Executing Python scripts
+
+A common use case is calling multiple blocks of Python code from Julia interactively. This can be accomplished in PythonCall via the [@pyexec](@ref) macro. For example, the sentence parsing application in the [Getting started](@ref py_getting_started) section could be rewritten as:
+
+```julia-repl
+julia> @pyexec """
+           global re
+           import re
+
+           def my_sentence(s):
+               words = re.findall("[a-zA-Z]+", s)
+               sentence = " ".join(words)
+               return sentence
+           """ => my_sentence
+Python: <function my_sentence at 0x7d83bb1b01a0>
+
+julia> sentence = my_sentence("PythonCall.jl is very useful!")
+Python: 'PythonCall jl is very useful'
+```
+
+Note the use of the `global` keyword to make the `re` package accessible in global scope, and the `=> my_sentence` syntax to create a Julia function named `my_sentence` that calls to the Python function of the same name. This syntax also supports calling to multiple functions and passing data back-and-forth:
+
+```julia-repl
+julia> @pyexec (num=10) => """
+           def add(a, b):
+             return a + b
+
+           def subtract(a, b):
+               return a - b
+
+           plusone = num + 1
+           """ => (add, subtract, plusone::Float64)
+(add = <py function add at 0x721b001a7cc0>, subtract = <py function subtract at 0x721b001a7d70>, plusone = 11.0)
+
+julia> add(4, 3)
+Python: 7
+
+julia> subtract(4, 3)
+Python: 1
+
+julia> plusone
+11.0
+```
+
+Here we demonstrate passing a named variable, `num`, via the use of the `=>` syntax again, and returning named output, with the last element, `plusone`, being cast to a Julia object via the `::` syntax. See [@pyexec](@ref), [@pyeval](@ref), and their functional forms [pyexec](@ref) and [pyeval](@ref), for more.
+
 ## Conversion between Julia and Python
 
 A Julia object can be converted to a Python one either explicitly (such as `Py(x)`) or