[doc][release_notes] update release notes for boost 1.57:
* add new algorithm num_segments
* add resolved bug #10345
* remove "." at end of line
[doc][release notes] remove num_segments from the new functionality section
since boost 1.57 is already closed for new features (it has to wait for boost 1.58)
Merge pull request #164 from mkaravel/fix/release_notes

[doc][release_notes] update release notes for boost 1.57: