Add notes about predefined strings not available to plugins

The documention refers dissector authors to helpful predifined string structures
that plugin authors unfortunately cannot use.

Bug: 13828
Change-Id: I62cdfeb200c9b354aed44d40c80a0e8f9e8f910b
Reviewed-on: https://code.wireshark.org/review/22339
Reviewed-by: Michael Mann <mmann78@netscape.net>

doc/README.dissector

@@ -1023,7 +1023,7 @@ For fields with that unit name, you would declare a "unit_name_string":
 This is typically the case when abbreviations are used instead of full words.)
 
 There are several "common" unit name structures already defined in
-epan/unit_strings.h.  Dissector authors may choose to add the unit name
+epan/unit_strings.h. Dissector authors may choose to add the unit name
 structure there rather than locally in a dissector.
 
 For hf[] array FT_(U)INT*, FT_FlOAT and FT_DOUBLE fields that need a
@@ -1091,6 +1091,12 @@ in tfs.h, included via packet.h.
 Custom fields (BASE_CUSTOM) should use CF_FUNC(&custom_format_func) for the
 'strings' field.
 
+-- Note to plugin authors
+Data cannot get exported from DLLs. For this reason plugin authors cannot use
+existing fieldconvert strings (e.g. from existing dissectors or those from
+epan/unit_strings.h). Plugins must define value_strings, unit_name_strings,
+range_strings and true_false_strings locally.
+
 bitmask (BITMASK)
 -----------------
 If the field is a bitfield, then the bitmask is the mask which will