add missing description for +SDio option

[vinoski]

Add explanatory text for the +SDio option, which is used to set the number
of dirty I/O schedulers.

[OTP-Maintainer]

Unable to build docs: mplemented, yet! Some font data in the AFM file will be ignored.
Support for 'StartTrackKern' has not been implemented, yet! Some font data in the AFM file will be ignored.
Support for 'StartTrackKern' has not been implemented, yet! Some font data in the AFM file will be ignored.
Support for 'StartTrackKern' has not been implemented, yet! Some font data in the AFM file will be ignored.
Exception
org.apache.fop.apps.FOPException: Input/output error
java.io.IOException: Input/output error

make[4]: *** [../pdf/wx-1.2.pdf] Error 1
make[4]: Leaving directory /ldisk/jenkins/workspace/pullrequests/otp/lib/wx/doc/src' make[3]: *** [release_docs] Error 2 make[3]: Leaving directory/ldisk/jenkins/workspace/pullrequests/otp/lib/wx/doc/src'
make[2]: *** [release_docs] Error 2
make[2]: Leaving directory /ldisk/jenkins/workspace/pullrequests/otp/lib/wx' make[1]: *** [release_docs] Error 2 make[1]: Leaving directory/ldisk/jenkins/workspace/pullrequests/otp/lib'
make: *** [release_docs] Error 2

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I am a script, I am not human

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[OTP-Maintainer]

Patch has passed first testings and has been assigned to be reviewed

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I am a script, I am not human

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