issue in the doc

Page:
https://www.postgresql.org/docs/current/textsearch-intro.html

The documentation says:

tsvector @@ tsquery
tsquery @@ tsvector
text @@ tsquery
text @@ text

The first two of these we saw already. The form text @@ tsquery is
equivalent to to_tsvector(x) @@ y. The form text @@ text is equivalent
to to_tsvector(x)
@@ plainto_tsquery(y).

Question:

In the expression:
to_tsvector(x) @@ plainto_tsquery(y)

what do x and y refer to exactly?

are they correspond to the left-hand and right-hand sides of the original
expression (text @@ text) ?

if yes , i think it's better to mention the order of the x and y like :
(x @@ y ) or ( y @@ x)

thanks