net/http: document Content-Type sniffing

[dsymonds]

The mechanism and limits of the Content-Type sniffer in the http package should be
documented and visible in "godoc http".

[dsymonds]

Comment 1:

Labels changed: removed priority-medium.

[rsc]

Comment 3:

Labels changed: added priority-go1.

[robpike]

Comment 4:

Owner changed to builder@golang.org.

[rsc]

Comment 6:

Labels changed: added go1-must.

[rsc]

Comment 7:

Owner changed to @rsc.

Status changed to Started.

[rsc]

Comment 8:

This issue was closed by revision d9da346.

Status changed to Fixed.