@tylernz. I'm not familiar with TextMate, although I am with other IDEs.
Unlike other IDEs, ST cannot produce a tooltip, or prompt box - the nearest we have is what appears in the completions list. I could have included 'string ' etc. in my completions file but decided against it. Besides, the list isn't wide enough to show all this information. My strpos completion appears like this:
strpos(haystack, needle, offset])
There are help systems available via PackageControl that can jump to, or display, help on a key-combination. I've even created two myself - one just recently in another thread, which opens a .chm file at the relevant page. Some help systems will display the help in a panel at the bottom of the view, whereas other systems will search the web.