While these are 1 sentence, they certainly would not fit on one line in the book.
Symbol
1 minute casting time.
And if the creature in question has inscribed the symbol somewhere, I do expect to find details of that in the adventure.
While I agree that any uses of Symbol set up in an adventure ahead of time would be described, just stating "1 minute casting time" does not help a DM who wants to use it to set another trap.
Teleport
The creature teleports to the teleportation circle in area X/ to place Y where it has object Z from.
Even teleporting to somewhere you can see has a 25% chance to not be on target. So it is not likely to see use other than for escape.
While uses of teleport in combat are probably limited to escape, the description seems to require a teleportation circle or object, when the spell does not.
Thaumaturgy
The creature causes a minor, harmless wonder, such as rumbling earth or booming voice.
The list of effects possible is limited to 1 of 6 specific effects. It does not let the caster do any minor, harmless wonder. It also allows up to 3 to be running at a time, each for up to a minute.
True Polymorph
I'm not sure if this can be summarised, mostly because it has three distinct uses. It is possible to summarise any one of those and hopefully the adventure writer had an idea for giving this creature one of the spells from the highest level in existence.
Yeah, I don't think it really can be effectively summarized in one sentence either.
I really don't think many of the spells could accurately be summarized in such a way that a DM could look at it and use the spell with the fluency that the caster who actually knew the spell would be able. After going through several pages of the PHB, my best guess is that less than 40% of the spells could be effectively communicated in two lines. It just seems like something gets left out or misconstrued in short descriptions of spells so that the result can be quite different from the actual spell.
Are you also of the opinion that an adventure should not reprint the stats of monsters found in the Monster Manual? Since it's equally easy to look those up as it is to look up spells. I'm honestly interested, because to me they are equal.
Absolutely the are equal, and if you look at HotDQ, the MM creatures are not in the book. The only things in the appendices are the backgrounds, NPCs, creatures and magic items that are unique to the story.
Though, instead of spell descriptions, I could also use a good overview of tactics the spellcaster is likely to use. That tells me that I only need to familiarise myself with the spells there.
That would be really useful. I imagine those would be in the description of the encounter. Tactics would change somewhat from encounter to encounter.
It kind of fell off the previous page of this thread, but I made a spell card generator. You have to type in the spells you need once, but after that you can print them out and place them on a page behind the page that holds the stat block for the creature. It's not the same as being inline, but for people like me, it helps.
http://www.enworld.org/forum/showthread.php?361942-Make-your-own-spell-cards