Add the word "slow" to preserved unreferenced resources docs (fixes #380)
Add the word "slow" to preserved unreferenced resources docs (fixes #380)

This makes it more likely for someone who notices a big slow-down in
page splitting to discover the option.

qpdf/qpdf,jberkenbilt/qpdf,jberkenbilt/qpdf,jberkenbilt/qpdf,qpdf/qpdf,qpdf/qpdf,jberkenbilt/qpdf,qpdf/qpdf,qpdf/qpdf,jberkenbilt/qpdf