for example i have an xpath and wish to add a comment near it to identify it.
/html/body/div/table/tr/td/a{this is a link}
Not a comment syntax, but you can give string literals as predicate, which evaluates as true (imho) and should not change the outcome of the expression. I don't know if this has big performance drawbacks.
/html/body/div/table["this is"]["a table"]/tr/td/a["this is a link"]
But like mjv said, I also would stick to the syntax of the host language.