Documentation about range of allowed values of SDF parameters collision#ode_mu and collision#ode_mu2 is incorrect

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/149)) by Silvio Traversaro (Bitbucket: [traversaro](https://bitbucket.org/%7B34f404cb-5642-4f27-a032-e04c7143d776%7D/)).**

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The SDF format specification constraints the `mu` and `mu2` parameters to be in the `[0 ... 1]` range:
~~~
Coefficient of friction in the range of [0..1]
~~~
See https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/annotate/f2b9358f5fd94c5d2bcc1875d4180bcc964f4be1/sdf/1.6/surface.sdf?at=default&fileviewer=file-view-default#surface.sdf-46 .

However, the tutorial on friction states that any positive value is allowed for the mu and mu2 parameters : http://gazebosim.org/tutorials?tut=friction .