Update ``popen_handle`` term description

[AArdeev]

Product: Tarantool
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/popen/
SME: @ Totktonada

Details

• Currently, the document does not provide a consistent and transparent explanation of the popen_handle term.
• With that, a reader may get a confused or incorrect understanding.
• We need to define clearly what meaning does the popen_handle have and change the description in the article in such a way, that this meaning is provided transparently and consistently to the reader.
• As a result, the reader will gain a better understanding of the document and of the product.