Это не могло бы ответить точно на Ваш вопрос, но здесь является редактором, позволяет иметь более высокое представление кода: http://webpages.charter.net/edreamleo/front.html
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.
2019 edit
As pointed out in @Sepster's reply and elsewhere, starting with XPath 2.0, comments became possible with their cute "smiley face"-looking syntax. I'm only about 10 years late in editing this reply to mention very useful fact ;-)
Original reply c. 2009 (assumed XPATH 1.0)
No, the XPATH syntax doesn't allow to embed comments within the path string.
This is typically not a significant limitation because paths are usually short and a comment can be placed nearby, in the particular syntax of the host language (XSLT, C#, whatever...)