Fix the incorrect comment for set_blocking()

[tz-arm]

Description

Fix the incorrect comment for set_blocking() which impact the online API document.
Before:
* set_blocking(false) is equivalent to set_timeout(-1)
* set_blocking(true) is equivalent to set_timeout(0)
After:
* set_blocking(false) is equivalent to set_timeout(0)
* set_blocking(true) is equivalent to set_timeout(-1)

Pull request type

[ ] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[x] Docs update
[ ] Test update
[ ] Breaking change

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3509
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8580/

Triggering tests

/morph test
/morph export-build
/morph mbed2-build

[mbed-ci]

Test : FAILURE

Build number : 3294
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8580/3294

[mbed-ci]

Exporter Build : SUCCESS

Build number : 3128
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8580/

[0xc0170]

/morph test

[mbed-ci]

Test : SUCCESS

Build number : 3296
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8580/3296