docs: improve docs on pinning an extension repo

[nicks]

Hello @landism, @lianmakesthings,

Please review the following commits I made in branch nicks/private:

b40d83a020d5890007977fd4e9e7b92ac477542d (2022-02-18 19:31:53 -0500) docs: improve docs on pinning an extension repo see discussion in: https://github.com/tilt-dev/tilt/issues/5522

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