Use consistent language for string termination (#806)

Avoid hyphen, prefer space: "null terminated"

modules/ca/src/client/CAref.html

@@ -2751,7 +2751,7 @@ time.</p>
 <h4>Arguments</h4>
 <dl>
   <dt><code>PVNAME</code></dt>
-    <dd>A nil terminated process variable name string. EPICS process control
+    <dd>A null terminated process variable name string. EPICS process control
       function block database variable names are of the form "&lt;record
       name&gt;.&lt;field name&gt;". If the field name and the period separator
       are omitted then the "VAL" field is implicit. For example "RFHV01" and

modules/ca/src/client/cadef.h

@@ -465,7 +465,7 @@ typedef unsigned capri;
  *
  * See `caExample.c` in the example application created by `makeBaseApp.pl`.
  *
- * \param[in] pChanName A nil terminated process variable name string.
+ * \param[in] pChanName A null terminated process variable name string.
  * \parblock
  *  EPICS process control * function block database variable names are of the
  *  form `<record name>.<field name>`. If the field name and the period

modules/ca/src/client/db_access.h

@@ -128,7 +128,7 @@ LIBCA_API extern const int epicsTypeToDBR_XXXX [lastEpicsType+1];
 LIBCA_API extern const epicsType DBR_XXXXToEpicsType [LAST_BUFFER_TYPE+1];
 
 /* values returned for each field type
- *      DBR_STRING      returns a NULL terminated string
+ *      DBR_STRING      returns a null terminated string
  *      DBR_SHORT       returns an unsigned short
  *      DBR_INT         returns an unsigned short
  *      DBR_FLOAT       returns an IEEE floating point value