Limbaugh goes into a bit more depth, and also mentions the source for his claims:
They're all for it at Al-Qaeda. Wiping out gays? A bunch of attaboys
for doing that. However, the [Inspire Guide: Orlando Operation]
"suggests to 'avoid targeting places and crowds where minorities are
generally found in America,' because 'the federal government will be
the one taking full responsibility,'" meaning if you guys go out and
you attack a bunch of minorities, we won't get credit for it. The
American government will blame Republicans and conservatives and the
Tea Party, and we won't get credit. source
Here is the full quote from the guide:
The executer specifically chose a homosexual nightclub, and
even though the killing of such people is the most binding duty and
closer to human nature, but better than this is to avoid targeting
areas where minorities are found. This is in order not to
deviate the essence of the operation and letting it be
termed as a small issue as the American media is trying to
portray in the case of Mateen. The Western media focused on
the testimony by Mateen›s father who said that his son hates
homosexuals and that terrorist ideas had no place in his motives. The
media reiterated this, saying that Umar saw some homosexuals kissing
each other and that such a scene offended him. The media tried to
portray the operation motives to be against a particular group of
people in order to turn the American public away from the real
motives of the operation.
Most of the individuals present at
pulse nightclub were Latino. We recommend – And Allah knows best -
that it is better to avoid targeting places and crowds where
minorities are generally found in America. knowingly that the
federal government will be the one taking full responsibility.
Therefore we advise targeting areas where the Anglo-Saxon community
is generally concentrated. This class of the American community is
the majority and it is the one that is in the American leadership
So if we assume that the document is genuine[*], and if we assume that the Lone Jihad Guide Team belongs to Al-Qaeda[**], then yes, there has been a suggestion to avoid targeting areas where "minorities are found".
But the reason isn't that the "American government finds nothing wrong with [killing whites]", or that "Democrats will think Republicans where responsible for the attack", but that it distracts from the main message.
It also doesn't appear that ISIS had anything to do with the document. The document also did not say to not attack or kill minorities, but to not target them specifically. The document also does not mention the second amendment, gun rights, or conservatives.
The ADL summarizes the reason for avoiding targeting minorities like this:
[T]he pamphlet suggested that it would be best for future
perpetrators not to target specific groups in society, such as
Latinos or the LGBT community, because the focus of news coverage
would then be on the group targeted, rather than on the overall
terrorist element of the attack. source
[*] The document contains the logo of Al Malahem media, which according to the ADL did release the document, and is indeed associated with AQAP. At the moment, the blog post by the ADL is the most reputable source I could find referencing the document.
[**] The first google hit for Lone Jihad Guide Team is the linked document, and no reputable source links it to Al-Qaeda, but as mentioned above, the media that released the document is tied to Al-Qaeda.