doc: improve N-API string-to-native doc

Mention null termination handling in the string size accounting and
string truncation.

PR-URL: nodejs#35322
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michael Dawson <midawson@redhat.com>

doc/api/n-api.md

@@ -3080,9 +3080,10 @@ napi_status napi_get_value_string_latin1(napi_env env,
 * `[in] env`: The environment that the API is invoked under.
 * `[in] value`: `napi_value` representing JavaScript string.
 * `[in] buf`: Buffer to write the ISO-8859-1-encoded string into. If `NULL` is
-  passed in, the length of the string (in bytes) is returned.
+  passed in, the length of the string in bytes and excluding the null terminator
+  is returned in `result`.
 * `[in] bufsize`: Size of the destination buffer. When this value is
-  insufficient, the returned string will be truncated.
+  insufficient, the returned string will be truncated and null-terminated.
 * `[out] result`: Number of bytes copied into the buffer, excluding the null
   terminator.
 
@@ -3109,9 +3110,10 @@ napi_status napi_get_value_string_utf8(napi_env env,
 * `[in] env`: The environment that the API is invoked under.
 * `[in] value`: `napi_value` representing JavaScript string.
 * `[in] buf`: Buffer to write the UTF8-encoded string into. If `NULL` is passed
-  in, the length of the string (in bytes) is returned.
+  in, the length of the string in bytes and excluding the null terminator is
+  returned in `result`.
 * `[in] bufsize`: Size of the destination buffer. When this value is
-  insufficient, the returned string will be truncated.
+  insufficient, the returned string will be truncated and null-terminated.
 * `[out] result`: Number of bytes copied into the buffer, excluding the null
   terminator.
 
@@ -3137,9 +3139,10 @@ napi_status napi_get_value_string_utf16(napi_env env,
 * `[in] env`: The environment that the API is invoked under.
 * `[in] value`: `napi_value` representing JavaScript string.
 * `[in] buf`: Buffer to write the UTF16-LE-encoded string into. If `NULL` is
-  passed in, the length of the string (in 2-byte code units) is returned.
+  passed in, the length of the string in 2-byte code units and excluding the
+  null terminator is returned.
 * `[in] bufsize`: Size of the destination buffer. When this value is
-  insufficient, the returned string will be truncated.
+  insufficient, the returned string will be truncated and null-terminated.
 * `[out] result`: Number of 2-byte code units copied into the buffer, excluding
   the null terminator.