doc: write Go 1.15 release notes

[dmitshur]

Tracking bug for writing the Go 1.15 Release Notes.

The latest state on tip can be viewed at https://tip.golang.org/doc/go1.15.

Previously #36878, #17929, #15810, #5929, etc.

[gopherbot]

Change https://golang.org/cl/227997 mentions this issue: doc/go1.15, net/url: document new method URL.Redacted

[ainar-g]

I think, the changes to the database/... packages should be documented as well:

+pkg database/sql, method (*DB) SetConnMaxIdleTime(time.Duration)
+pkg database/sql, method (*Row) Err() error
+pkg database/sql, type DBStats struct, MaxIdleTimeClosed int64
+pkg database/sql/driver, type Validator interface { IsValid }
+pkg database/sql/driver, type Validator interface, IsValid() bool

[gopherbot]

Change https://golang.org/cl/235580 mentions this issue: doc/go1.15: document no language changes

[gopherbot]

Change https://golang.org/cl/235757 mentions this issue: doc: update Go 1.15 release notes using relnote

[gopherbot]

Change https://golang.org/cl/236157 mentions this issue: doc/go1.15: add release notes for strconv

[gopherbot]

Change https://golang.org/cl/236158 mentions this issue: doc/go1.15: add release notes for time

[gopherbot]

Change https://golang.org/cl/236159 mentions this issue: doc/go1.15: release notes for new linker