[string.io] Rephrase sentry conversion

Author: jensmaurer

source/strings.tex

@@ -3550,10 +3550,10 @@
 \pnum
 \effects
 Behaves as a formatted input function\iref{istream.formatted.reqmts}.
-After constructing a
-\tcode{sentry}
-object, if the sentry converts to \tcode{true}, calls
-\tcode{str.erase()}
+After constructing a \tcode{sentry} object,
+if the \tcode{sentry} object returns \tcode{true}
+when converted to a value of type \tcode{bool},
+calls \tcode{str.erase()}
 and then extracts characters from \tcode{is} and appends them
 to \tcode{str} as if by calling
 \tcode{str.append(1, c)}.
@@ -3630,10 +3630,10 @@
 Behaves as an unformatted input function\iref{istream.unformatted},
 except that it does not affect the value returned by subsequent calls to
 \tcode{basic_istream<>::gcount()}.
-After constructing a
-\tcode{sentry}
-object, if the sentry converts to \tcode{true}, calls
-\tcode{str.erase()}
+After constructing a \tcode{sentry} object,
+if the \tcode{sentry} object returns \tcode{true}
+when converted to a value of type \tcode{bool},
+calls \tcode{str.erase()}
 and then extracts characters from \tcode{is} and appends them
 to \tcode{str} as if by calling
 \tcode{str.append(1, c)}