Docs: systr add docs to basic internal procs

Author: ZoomRmc

lib/system/sysstr.nim

@@ -48,18 +48,30 @@ else:
     cast[NimString](newObjNoInit(addr(strDesc), size))
 
 proc rawNewStringNoInit(space: int): NimString =
+  ## Returns a newly-allocated NimString with `reserved` set.
+  ## .. warning:: `len` and the terminating null-byte are not set!
   let s = max(space, 7)
   result = allocStrNoInit(sizeof(TGenericSeq) + s + 1)
   result.reserved = s
   when defined(gogc):
     result.elemSize = 1
 
 proc rawNewString(space: int): NimString {.compilerproc.} =
+  ## Returns a newly-allocated and *not* zeroed NimString
+  ## with everything required set:
+  ## - `reserved`
+  ## - `len`
+  ## - terminating null-byte
   result = rawNewStringNoInit(space)
   result.len = 0
   result.data[0] = '\0'
 
 proc mnewString(len: int): NimString {.compilerproc.} =
+  ## Returns a newly-allocated and zeroed NimString
+  ## with everything required set:
+  ## - `reserved`
+  ## - `len`
+  ## - terminating null-byte
   result = rawNewStringNoInit(len)
   result.len = len
   zeroMem(addr result.data[0], len + 1)
@@ -217,13 +229,16 @@ proc appendChar(dest: NimString, c: char) {.compilerproc, inline.} =
 
 proc setLengthStr(s: NimString, newLen: int): NimString {.compilerRtl.} =
   let n = max(newLen, 0)
-  if s == nil:
-    return if n == 0: s else: mnewString(n)
-  elif n <= s.space:
-    result = s
+  if s == nil: # early return check
+    if n == 0:
+      return s
+    else:
+      return mnewString(n) # sets everything required
+  if n <= s.space:
+    result = s # len and null-byte need updating
   else:
     let sp = max(resize(s.space), n)
-    result = rawNewStringNoInit(sp)
+    result = rawNewStringNoInit(sp) # len and null-byte not set
     copyMem(addr result.data[0], unsafeAddr(s.data[0]), s.len)
     zeroMem(addr result.data[s.len], n - s.len)
   result.len = n