[CLA Approved] Fix references to telemetry data "unit" in the plot LEGEND form, top legend display and documentation (#5546)

* [Plot] Rename the `units` attribute to `unit` (#5540)

Rename the `units` attribute to `unit` when referencing the telemetry
data "unit" within:
- the legend-form vue for setting the `valueToShowWhenCollapsed` and
  `showUnitsWhenExpanded` and displaying the available choices in write
  mode,
- the plot-options-browse vue for displaying the selected choices
  within the form in read mode,
- the plot-legend-item-collapsed when checking which field to display.

* [Documentation] Update API documentation (#3562)

API.md

@@ -390,7 +390,7 @@ A telemetry object is a domain object with a telemetry property.  To take an exa
             {
                 "key": "value",
                 "name": "Value",
-                "units": "kilograms",
+                "unit": "kilograms",
                 "format": "float",
                 "min": 0,
                 "max": 100,
@@ -425,7 +425,7 @@ attribute      | type   | flags    | notes
 `name`         | string | optional | a human readable label for this field.  If omitted, defaults to `key`.
 `source`       | string | optional | identifies the property of a datum where this value is stored.  If omitted, defaults to `key`.
 `format`       | string | optional | a specific format identifier, mapping to a formatter.  If omitted, uses a default formatter.  For enumerations, use `enum`.  For timestamps, use `utc` if you are using utc dates, otherwise use a key mapping to your custom date format.  
-`units`        | string | optional | the units of this value, e.g. `km`, `seconds`, `parsecs`
+`unit`        | string | optional | the unit of this value, e.g. `km`, `seconds`, `parsecs`
 `min`          | number | optional | the minimum possible value of this measurement.  Will be used by plots, gauges, etc to automatically set a min value.
 `max`          | number | optional | the maximum possible value of this measurement.  Will be used by plots, gauges, etc to automatically set a max value.
 `enumerations` | array  | optional | for objects where `format` is `"enum"`, this array tracks all possible enumerations of the value.  Each entry in this array is an object, with a `value` property that is the numerical value of the enumeration, and a `string` property that is the text value of the enumeration.  ex: `{"value": 0, "string": "OFF"}`.  If you use an enumerations array, `min` and `max` will be set automatically for you.
@@ -1082,4 +1082,4 @@ View provider Example:
         return openmct.priority.HIGH;
     }
 }
-```
+```

src/plugins/plot/inspector/PlotOptionsBrowse.vue

@@ -136,7 +136,7 @@
                     <span v-if="showValueWhenExpanded">Value</span>
                     <span v-if="showMinimumWhenExpanded">Min</span>
                     <span v-if="showMaximumWhenExpanded">Max</span>
-                    <span v-if="showUnitsWhenExpanded">Units</span>
+                    <span v-if="showUnitsWhenExpanded">Unit</span>
                 </div>
             </li>
         </ul>

src/plugins/plot/inspector/forms/LegendForm.vue

@@ -54,7 +54,7 @@
                 <option value="nearestValue">Nearest value</option>
                 <option value="min">Minimum value</option>
                 <option value="max">Maximum value</option>
-                <option value="units">Units</option>
+                <option value="unit">Unit</option>
             </select>
         </div>
     </li>
@@ -89,7 +89,7 @@
                     v-model="showUnitsWhenExpanded"
                     type="checkbox"
                     @change="updateForm('showUnitsWhenExpanded')"
-                > Units</li>
+                > Unit</li>
             </ul>
 
         </div>

src/plugins/plot/legend/PlotLegendItemCollapsed.vue

@@ -41,7 +41,7 @@
         <span class="plot-series-name">{{ nameWithUnit }}</span>
     </div>
     <div
-        v-show="!!highlights.length && (valueToShowWhenCollapsed !== 'none' && valueToShowWhenCollapsed !== 'units')"
+        v-show="!!highlights.length && (valueToShowWhenCollapsed !== 'none' && valueToShowWhenCollapsed !== 'unit')"
         class="plot-series-value hover-value-enabled"
         :class="[{ 'cursor-hover': notNearest }, valueToDisplayWhenCollapsedClass, mctLimitStateClass]"
     >