docs: add channel priority definition documentation

[ruben-arts]

Document this logic:

[project]
name = "test_channel_priority"
platforms = ["linux-64"]
channels = ["conda-forge"]

[feature.a]
channels = ["nvidia"]

[feature.b]
channels = [ "pytorch", {channel = "nvidia", priority = 1}]

[feature.c]
channels = [ "pytorch", {channel = "nvidia", priority = -1}]

[environments]
a = ["a"]
b = ["b"]
c = ["c"]