MINOR: h2: explain committed_extra_streams dec on h2_init() error

MINOR: h2: explain committed_extra_streams dec on h2_init() error

h2_init() is now responsible to increment committed_extra_streams for
new frontend connections, in relation to the newly implemented
stream-elasticity feature. In case of an early error, a mirroring
decrement is executed on fail_stream label.

However, for now this error label can only be selected via BE conns. In
fact, it's not yet possible for h2_init() to fail after the extra
streams increment.

However, the decrement operation is kept to prevent any omissions in
case of future evolutions of h2_init() error path. To prevent reporting
of a possible dead code, add an extra comment which summarizes the
situation.