there is information in the DM Guide
I read it as the weapon damage is doubled but any bonuses from say strength are added after the base damage is doubled and not also doubled.
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Critical Hits
If a player slides a natural 20 (not when a monster rolls a natural 20), the damage inflicted by the weapon is doubled. Bonus damage from special weapons (e.g., +2 SLING OF IMPACT vs. big foes) as well as “vulnerable” damage (e.g., Cold vs. red dragon) is also doubled on a natural 20.
Only Natural 20s Automatically Hit
Weapons with the keen or smiting property (inherently or via a spell or oil) can score a critical hit when the player slides a natural 19 or 20. An ultra keen/smiting weapon crits on a natural 18, 19, or 20. But, no matter what the crit range of the weapon is, only a natural 20 is an automatic hit. Even if the weapon has an expanded crit range, if it lands anywhere other than the 20 zone, you must still check to see if the attack succeeds before assigning damage.
Crit Immunity
In role-playing terms, a critical hit means the victim has been struck in a vital part of its anatomy. A creature lacking any discernible vital anatomy cannot be critically hit. Constructs, incorporeal creatures, oozes, plants, or undead are all examples of creatures without vital anatomy. In general, if a monster is immune to being sneak attacked, it’s also immune to critical hits. The room’s description will specify whether a specific monster makes an exception to this rule.
There are tokens which override this restriction. E.g., CHARM OF FINAL REPOSE allows the wearer to crit/sneak attack undead and AMULET OF THE TINKERER allows the wearer to crit/sneak attack constructs. But keep in mind, those items allow the one specific character using the item to crit that one specific creature type. They do not bestow their abilities to other characters or other creature types.
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*mental note* always listen to Jeff