Specification Message Body: Inconsistency size encrypted content in regular frame

The specification _aws-encryption-sdk-specification/data-format/message-body.md_ section _Framed Data_ subsection _Regular Frame_ contains the following requirement:

**Encrypted Content**
> The encrypted data for each frame, as returned by the encryption algorithm.
> The length of the encrypted content of a Regular Frame MUST be equal to the Frame Length.

This requirement states that the size of the encrypted content in one frame must equal to Frame Length. However Frame Length is defined as the amount of plain text which we want to encrypt in one regular frame. The size of the content which we want to encrypt does not have to be equal to the size of the encrypted content. Because of this the requirement is inconsistent.

Solutions would be adding a requirement to the encryption algorithm that the size of the plain text is equal to the encrypted content or we removing this requirement. Note that there should be some upper limit on the size of the encrypted content to prevent frames from getting too big.