Quotes and hyphens not removed by tm package functions while cleaning corpus

removePunctuation uses gsub('[[:punct:]]','',x) i.e. removes symbols: !"#$%&'()*+, \-./:;<=>?@[\\\]^_{|}~`. To remove other symbols, like typographic quotes or bullet signs (or any other), declare your own transformation function:

removeSpecialChars <- function(x) gsub("“•”","",x)
docs <- tm_map(docs, removeSpecialChars)

Or you can go further and remove everything that is not alphanumerical symbol or space:

removeSpecialChars <- function(x) gsub("[^a-zA-Z0-9 ]","",x)
docs <- tm_map(docs, removeSpecialChars)