spelling: call it null-terminate consistently

With a dash, using two Ls. Also for different forms of the word.

Use NULL in all uppercase if it means a zero pointer.

Follow-up to 307b7543ea

Closes #17489

docs/internals/STRPARSE.md

@@ -18,7 +18,7 @@ if(curlx_str_word(&line, &word1, MAX) ||
   fprintf(stderr, "ERROR\n");
 ~~~
 
-The input pointer **must** point to a null terminated buffer area or these
+The input pointer **must** point to a null-terminated buffer area or these
 functions risk continuing "off the edge".
 
 ## Strings

docs/libcurl/curl_easy_ssls_export.md

@@ -68,10 +68,10 @@ username/password are not exported.
 
 ## Session Key
 
-This is a printable, 0-terminated string that starts with **hostname:port**
-the session ticket is originating from and also contains all relevant
-SSL parameters used in the connection. The key also carries the name
-and version number of the TLS backend used.
+This is a printable, null-terminated string that starts with **hostname:port**
+the session ticket is originating from and also contains all relevant SSL
+parameters used in the connection. The key also carries the name and version
+number of the TLS backend used.
 
 It is recommended to only persist **session_key** when it can be protected
 from outside access. Since the hostname appears in plain text, it would

docs/libcurl/curl_formadd.md

@@ -79,7 +79,7 @@ contain zero-valued bytes.
 
 followed by a pointer to the contents of this part, the actual data to send
 away. libcurl copies the provided data, so your application does not need to
-keep it around after this function call. If the data is not null terminated,
+keep it around after this function call. If the data is not null-terminated,
 or if you would like it to contain zero bytes, you must set the length of the
 name with **CURLFORM_CONTENTSLENGTH**. The copied data is freed by
 curl_formfree(3).

docs/libcurl/curl_url_get.md

@@ -71,7 +71,7 @@ does not decode the scheme, the port number or the full URL.
 The query component also gets plus-to-space conversion as a bonus when this
 bit is set.
 
-Note that this URL decoding is charset unaware and you get a zero terminated
+Note that this URL decoding is charset unaware and you get a null-terminated
 string back with data that could be intended for a particular encoding.
 
 If there are byte values lower than 32 in the decoded string, the get

docs/libcurl/opts/CURLOPT_PREREQFUNCTION.md

@@ -58,14 +58,14 @@ This function is passed the following arguments:
 
 ## `conn_primary_ip`
 
-A null-terminated pointer to a C string containing the primary IP of the
+A pointer to a null-terminated C string containing the primary IP of the
 remote server established with this connection. For FTP, this is the IP for
 the control connection. IPv6 addresses are represented without surrounding
 brackets.
 
 ## `conn_local_ip`
 
-A null-terminated pointer to a C string containing the originating IP for this
+A pointer to a null-terminated C string containing the originating IP for this
 connection. IPv6 addresses are represented without surrounding brackets.
 
 ## `conn_primary_port`