as the test is small we do not want split up in multiple codegen units
and also as there is multiple test running at the same time this
will reduce the number of concurrent threads

Fixes a few repetitions of "like like" in the `trim*` methods documentation of `str`.

Update BufWriter example to include call to flush()

I was playing with a writing a Huffman encoder/decoder and was getting weird corruptions and truncations.  I finally realized it was was because `BufWriter` was swallowing write errors 😬.  I've found Rust to generally be explicit and err on the safe side, so I definitely found this unintuitive and not "rustic".

https://twitter.com/johnterickson/status/1159514988123312128

…lacrum

rustdoc use -Ccodegen-units=1 by default for test compile

as the test is small we do not want split up in multiple codegen units
and also as there is multiple test running at the same time this
will reduce the number of concurrent threads

tested the test time with `./x.py test src/libcore --doc`
for my 16 core 32 thread cpu i get about 6% faster execution
and my 2 core 4 thread cpu I get about 10% faster execution

cc rust-lang#63638
 r? @Mark-Simulacrum

Update sync condvar doc style

…schievink

Fix word repetition in str documentation

Fixes a few repetitions of "like like" in the `trim*` methods documentation of `str`.