cmd/compile: improve escape analysis explanation

No code changes, only revised comments in an attempt to make
escape analysis slightly less confusing.

Updates golang#23109.

Change-Id: I5ee6cea0946ced63f6210ac4484a088bcdd862fb
Reviewed-on: https://go-review.googlesource.com/121001
Run-TryBot: David Chase <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
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[release-branch.go1.10] go1.10.3

Change-Id: If2abf35c922737d48ec72881428dc3353b016f7f
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Run-TryBot: Filippo Valsorda <[email protected]>
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[release-branch.go1.9] go1.9.7

Change-Id: I417f9b711f92d0297d9d24272cb88365cdd945d3
Reviewed-on: https://go-review.googlesource.com/116497
Run-TryBot: Filippo Valsorda <[email protected]>
Reviewed-by: Filippo Valsorda <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

[release-branch.go1.10] go1.10.2

Change-Id: I3d88ab55277e4386177cda06e2eb5a9b0e26cf93
Reviewed-on: https://go-review.googlesource.com/110338
Run-TryBot: Andrew Bonventre <[email protected]>
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TryBot-Result: Gobot Gobot <[email protected]>

[release-branch.go1.9] go1.9.6

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Reviewed-on: https://go-review.googlesource.com/110336
Run-TryBot: Andrew Bonventre <[email protected]>
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[release-branch.go1.10] go1.10.1

Change-Id: I0636f60e705f8182d6fba4ce6199c67f94c04e36
Reviewed-on: https://go-review.googlesource.com/103168
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TryBot-Result: Gobot Gobot <[email protected]>

[release-branch.go1.9] go1.9.5

Change-Id: If8542fb3e9064971fb0b3be8f910fbfd5061010e
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[release-branch.go1.10] go1.10

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[release-branch.go1.10] go1.10rc2

Change-Id: I393ff9e8599d20f71545ab9f4191993f5d6fa5a4
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[release-branch.go1.9] go1.9.4

Change-Id: I2ab24e37b21d4b3275ffaeab3cb27d4e8e276e1e
Reviewed-on: https://team-review.git.corp.google.com/213171
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