I am also interested as I am cleared to return to work later this week. Basically what I find is that the AA website refers you to this statement:
"Instead of the negative COVID-19 test results, travelers also have the option of providing documentation of having had recovered from COVID-19 by showing proof of a recent positive viral test (in the 90 days preceding travel) and a letter from their healthcare provider or a public health official stating that they were cleared to travel. Travelers must also complete a travel attestation form provided by the airlines during boarding."
On the CDC site covering this discussion it states:
"A letter from your healthcare provider or a public health official that clears you to end isolation, e.g., to return to work or school, can be used to show you are cleared to travel, even if travel isn’t specifically mentioned in the letter"
So it does not seem that there is a defined form for your doctor to fill out but you do need your positive test report to establish a reference date.