misc: Update documentation of SimObject related APIs

Updated documentation of Drain, Serialize, Evnet queue and Simobject
APIs. Made some corrections to where the documentation was available
in the code but did not appear in the documentation.

Change-Id: I5254e87eb5663232e824bcd5592da0a04eba673b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/31814
Reviewed-by: Ayaz Akram <yazakram@ucdavis.edu>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/sim/serialize.cc

@@ -278,19 +278,45 @@ CheckpointIn::~CheckpointIn()
 {
     delete db;
 }
-
+/**
+ * @param section Here we mention the section we are looking for
+ * (example: currentsection).
+ * @param entry Mention the entry we are looking for (example: interrupt
+ * time) in the section.
+ *
+ * @return Returns true if the entry exists in the named section
+ * we are looking in.
+ */
 bool
 CheckpointIn::entryExists(const string &section, const string &entry)
 {
     return db->entryExists(section, entry);
 }
-
+/**
+ * @param section Here we mention the section we are looking for
+ * (example: currentsection).
+ * @param entry Mention the entry we are looking for (example: Cache
+ * line size etc) in the section.
+ * @param value Give the value at the said entry.
+ *
+ * @return Returns true if the searched parameter exists with
+ * the value, given the section .
+ */
 bool
 CheckpointIn::find(const string &section, const string &entry, string &value)
 {
     return db->find(section, entry, value);
 }
-
+/**
+ * @param section Here we mention the section we are looking for
+ * (example: currentsection).
+ * @param entry Mention the SimObject we are looking for (example:
+ * interruput time) in the section.
+ * @param value Give the value at the said entry.
+ *
+ * @return Returns true if a SimObject exists in the section.
+ *
+ */
 bool
 CheckpointIn::findObj(const string &section, const string &entry,
                     SimObject *&value)