Two parts here. The first is that yes, the comment field will never show in the alternative text, as it seemed that most people were using them for internal metadata (mostly but not entirely credits). But the second is that the descriptive text shouldn't be longer, it should say everything you want to convey. In this case, it sounds like what you want to convey is "Dejected" or "Spock" or "Reading", or possibly "Dejected: sigh". The only reason to put in a longer description if you want the icon to convey more information.
Keywords are displayed if the description field is left blank, so if you just want to leave the description field entirely blank, then the keyword is what will show up. So if you are happy with "sigh" as the alternative text, just leave the description field entirely blank. If you have multiple keywords for a single icon, the one you used will show up (or all of them, if you didn't use a keyword to select that icon).
In other words, if all the information they contain is described by their single keyword, then you are golden, and you don't need to use the description field at all.
But you also want to avoid using lots of specialized punctuation: "{{{}}}", or "!" as a delimiter in your alternative text. For people using text to speech, that punctuation is most likely to be completely ignored, and if it's read aloud, might have an effect you don't expect. If you are using very widely used special punctuation (e.g. "*hugs*"), then there is at least a chance that the individual might have configured her text-to-speech functionality to make it clear the punctuation is there in some other way (e.g. speaking the text as if it were bolded). But out-of-the-box, Jaws said "{{{hugs}}}" as "left brace left brace left brace hugs right brace right brace right brace". The idea in alternative text is that it should be exactly and only what you want to convey, keeping in mind that it is very likely to be read aloud. Somebody upthread pointed out that this document should be amended to say "keep in mind that you can't really skim with text-to-speech, so you have to balance being descriptive with being brief.)
no subject
Keywords are displayed if the description field is left blank, so if you just want to leave the description field entirely blank, then the keyword is what will show up. So if you are happy with "sigh" as the alternative text, just leave the description field entirely blank. If you have multiple keywords for a single icon, the one you used will show up (or all of them, if you didn't use a keyword to select that icon).
In other words, if all the information they contain is described by their single keyword, then you are golden, and you don't need to use the description field at all.
But you also want to avoid using lots of specialized punctuation: "{{{}}}", or "!" as a delimiter in your alternative text. For people using text to speech, that punctuation is most likely to be completely ignored, and if it's read aloud, might have an effect you don't expect. If you are using very widely used special punctuation (e.g. "*hugs*"), then there is at least a chance that the individual might have configured her text-to-speech functionality to make it clear the punctuation is there in some other way (e.g. speaking the text as if it were bolded). But out-of-the-box, Jaws said "{{{hugs}}}" as "left brace left brace left brace hugs right brace right brace right brace". The idea in alternative text is that it should be exactly and only what you want to convey, keeping in mind that it is very likely to be read aloud. Somebody upthread pointed out that this document should be amended to say "keep in mind that you can't really skim with text-to-speech, so you have to balance being descriptive with being brief.)