doc: fix and expand explanation

[tshepang]

I see that extend() is not called if new_len > len()

[rust-highfive]

r? @alexcrichton

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[alexcrichton]

@bors: r+ d7a5aba

[alexcrichton]

@bors: rollup

[bors]

⌛ Testing commit d7a5aba with merge 914c4db...

[bors]

💔 Test failed - auto-mac-64-opt

[alexcrichton]

@bors: retry

On Thu, Oct 29, 2015 at 7:43 PM, bors [email protected] wrote:

[image: 💔] Test failed - auto-mac-64-opt
http://buildbot.rust-lang.org/builders/auto-mac-64-opt/builds/6927

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#29458 (comment).

[bors]

⚡ Previous build results for auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-win-msvc-32-opt are reusable. Rebuilding only auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-64-opt...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt