Ratio: Fix #6498+merge from trunk

svn path=/branches/release/boost/ratio/; revision=77692
Ratio: document fix for #6498

svn path=/branches/release/libs/ratio/; revision=77696
Ratio: mergre from trunk: fixes #7075

svn path=/branches/release/boost/ratio/; revision=79367
Merge changes from trunk:

Fix PDF install rule so that it's explicit and automatically invokes a PDF build when specified on the command line.
So "bjam pdfinstall" will now build and install the PDF to the current directory.
This works around some problems that the previous versions had if the user did not have an FO processor installed (basically Daniel James was unable to build the HTML docs for the distribution if the pdfinstall rule was implicit).

svn path=/branches/release/libs/ratio/; revision=79500
Ratio: merge from trunk

svn path=/branches/release/boost/ratio/; revision=80406
Ratio: merge 80896

svn path=/branches/release/boost/ratio/; revision=80920
Thread: Merge 80946,80947

svn path=/branches/release/boost/ratio/; revision=80963
Ratio: merge from trunk 1.53

svn path=/branches/release/boost/ratio/; revision=81665
Ratio: warning removal

svn path=/branches/release/libs/ratio/; revision=82016