[stringbuf.members] Minor fixes to P0408R7 wording.

"str()" should be "str"; we're talking about all str member functions here.
Add comma after "For efficiency reasons".
"i.e." -> "e.g." since we're describing an example case.

Author: Dawn Perchik

source/iostreams.tex

@@ -7697,7 +7697,7 @@
 Characters can be initialized by writing to the stream,
 by constructing the \tcode{basic_stringbuf}
 passing a \tcode{basic_string} argument, or
-by calling one of the \tcode{str()} member functions
+by calling one of the \tcode{str} member functions
 passing a \tcode{basic_string} as an argument.
 In the latter case, all characters initialized prior to the call
 are now considered uninitialized
@@ -7732,10 +7732,10 @@
 
 \pnum
 \begin{note}
-For efficiency reasons
+For efficiency reasons,
 stream buffer operations might violate invariants of \tcode{buf}
 while it is held encapsulated in the \tcode{basic_stringbuf},
-i.e., by writing to characters in the range
+e.g., by writing to characters in the range
 \range{\tcode{buf.data() + buf.size()}}{\tcode{buf.data() + buf.capacity()}}.
 All operations retrieving a \tcode{basic_string} from \tcode{buf}
 ensure that the \tcode{basic_string} invariants hold on the returned value.